Which Nailer is Right
Different nailers have different purposes. The first question most woodworkers ask when considering the purchase of a nailer, is which size to get. The answer to this question is dependant on the type of work the woodworker performs.

1. Small Crafts For doing small projects, your best bet is to use an 18 or 19 gauge brad nailer. These typically shoot 3/8" and longer brads. A pin nailer is about the same as a brad nailer, but shoots a headless pin. The primary benefits to using the smaller nailer for smaller projects is first of all, the ability to get it into tighter spots, and secondly, the smaller gauge nail produces less splitting. However, when using these small nailers for larger work, they do not have the driving power to pull workpieces together, and they have a greater chance for following the wood grain and blowing out.
2. Crafts and Light Cabinetry If your primary work is in larger crafts and the occasional small cabinet, then a 16 ga brad nailer may be best. This will typically shoot 5/8" to 2" nails, but blowout can occur with nails greater the 1 1/4" (Refer to the section below discussing blowout)
3. Cabinetry For full scale cabinetry, I would recommend a 15 ga finish nailer. The unit I have will shoot nails from 3/4" to 2", but 90% of the time I use 1 1/2" nails (4d).
4. Full Range If your work runs the full gambit from small jewelry boxes to large cabinetry, then I would recommend using a dedicated nailer for each type of work. For the full range of uses, I would recommend the 18 ga small nailer for detail work, and the 15 ga nailer as the cabinetry workhorse. The smaller gauge nailer can get into tighter places, and has less of a tendency to split the wood, while the heavier gauge nailer has better holding power, enough force behind the drive to pull workpieces tighter together, and has less tendency to blowout.

   Because of this mix, I bought a top-of-the-line 15 ga nailer, and the cheapest and physically smallest version of the 18 ga. Because I place less demand on the smaller nailer, I can justify buying the cheapest one available. At $80, if the nailer lasts me more than two years(which it has), it is worth the investment. For the larger nailer, being the workhorse of the shop, if it fails, I am out of work until it is repaired, so the cost is justified. 

Blowout
Having a nail blow out the side of your workpiece is not only extremely frustrating, but it is also a fairly common problem. Blowout is typically observed when nailing one piece of solid lumber to the edge of another piece. The nail tip will either blister the surface of the plywood, or come completely out of the edge. Before I began to do cabinetry full-time, I used a 16 ga finish nailer as the sole nailer in the "hobby-shop". This nailer was inexpensive and generally did an adequate job, but on one particular project I had horrendous blowout problems.
 
If you are getting a lot of blowout, don't feel bad. It is not necessarily your aim, nor the tilt of the nailer. The most significant cause for blowout is when the nail will follow the annular rings in wood. This is because there are areas of soft wood surrounded by harder wood. The nail has the tendency to follow the softer wood and not want to pass through the harder areas. This will actually bend the nail as it passes through the wood.

• Nail Tip Orientation A significant factor which causes a nail to track improperly through the wood is the orientation of the nail tip. Unlike standard nails, strip nails used in a nailer don't have a four sided point, they have a two sided point (a wedge). As the nail passes through the grain of the wood, it's like driving a car down a rutted road. Just as the tires will try to follow the ruts, so will the nail try to follow the grain. The wedge acts as a keel to keep the nail tip following the grain. If you turn the nail 90° to the curve of the annular rings, the "keel effect" is reduced, and the nail will cut through the rings instead of following them.
• Tilt of the Nailer Going back to the analogy of a rut in the road, it should be fairly obvious that it is easier to cross over a rut at a right angle than it is to cross nearly parallel. If you try to cross a rut that is nearly parallel to your direction of travel, the rut will catch the tire and pull you in. However, if you cross a rut at a right angle, you'll feel a bump, but it won't pull you into the direction of the rut. The same is true for a nail.

  If your nail blows out to the right hand side, the common tendency is to move the nailer as far left as possible, and then even tilt it to the right so you don't get a blowout on the left hand side. This actually makes things worse. What should be done is to move the nailer tip closer to the side of the blowout, and tip it away. By doing this, the nail will be closer to cutting the annular rings at a right angle. This concept goes against a woodworker's common instinct.

• Use the Nails Profile Most nails are not round nor square, they are oval or rectangular. This is a manufacturing design, but it must be dealt with during use. Just as an "I-Beam" has greater strength up and down than it does side to side, the nail will deflect in the direction it is thinnest. The smaller gauge nails (16,18,&,19 gauge) have a rectangular cross-section. This makes it easier to bend them side-to-side. Generally, the thin side of most smaller nails coincides with the direction of their wedge. However, on the larger nails, this isn't always the case. In this case, you need to decide which is going to make the nail bend more: It's cross-section, or the direction of the wedge. If you are shooting a nail into annular rings, the wedge will be predominate, but if you are shooting into particle board where random particles can deflect the nail, then the cross-section will be predominate.
• Know Your Nailer Most of the time, the smaller nailers which use nails fabricated from a stamped steel plate, the wedge and cross-section result in the nailer blowing out side-to-side with respect to the nailer's body. With the 15 gauge nailers where each nail is cut from a long wire, the orientation of the wedge is commonly opposite. So when I use my small nailer, I always hold the nailer perpendicular to the plywood's edge, but for the larger nailer, I always hold it parallel to the plywood's edge. I follow this rule regardless what type of material I am shooting into, as the nail could deflect for any reason.

  Let's use assembling a faceframe to a cabinet shell for example. With the 18 ga nailer where the wedge point (edge) is in-line with the strip of nails, I will hold the nailer perpendicular to the plywood. With the larger 15 gauge nailer, the wedge is not in line with the strip, and therefore, I hold the nailer parallel to the plywood shell

• Nail Strength To state the obvious, a smaller nail will be more prone to bending than a larger nail. This is the number one reason why I limit the length of nail for each size (gauge) of nailer I use. The 18 gauge nail is too soft to track well in even the most homogeneous of materials beyond one inch in length. A 16 gauge nail will be prone to frequent blowouts when longer than 1 1/4 inches, and the same is true for 15 gauge nails past 2 inches.  
• Wood Grain Direction Many woodworkers, including professionals, don't consider the direction of the wood grain when driving a nail, but this can be one of the most difficult to detect causes of blowout. As stated before, if the annular rings of the wood are near perpendicular to the direction of the nail, it will follow the direction of the rings. Before driving your nail, examine the end-grain of your board. First off, this will give you a good indication as to whether you may have a blowout problem before it happens. Secondly, by knowing which direction the blowout might occur, you can take preemptive action to stop the blowout from happening. And finally, you may be able to orient the board differently to reduce the impact of a blowout (limiting the blowout to a non-visible side)  

Case Situation Back when I was still using a 16 gauge nailer, I shot a half a dozen nails into a faceframe, and every single one of them blow out the side. After the second nail blew out, I started moving the nailer further away from the blowout; but they still blew out. I was finally shooting 1/16 of an inch from the opposite side, and they still blew out the other side (some came out at nearly 90 degrees). Realizing the cause was the grain direction, I started shooting very close to the same side as the blow out, but tilted the gun as far off vertical as I possibly could, and they stopped blowing out. This was because I was cutting through the annular rings with the nail instead of following them.   

Air Pressure The last item which can cause, or reduce, blowout is the air pressure delivered to the nailer. Just as a bullet does 90% of its damage when it slows to 10% of its muzzle velocity, the nail will cause most of its damage (and deflection) as it slows down. Let me explain that with an example.

Let's say you have a "pump-type" BB gun and a couple of aluminum cans. Give the BB gun a single pump and shoot at the first can. The BB wouldn't penetrate the can, but it would nock it over and also make a good sized dent in the can. Now give the gun ten pumps and shoot the second can. This time, the can not only didn't fall over, but the BB made a tiny hole, and went straight through. Furthermore, if you compare the entrance and exit hole, the exit hole is larger than the entrance isn't it? (A BB is solid copper and doesn't deform, so this isn't because it squashed out.) The reason is because the BB transfered more of it's energy to the can when it was moving slower.

With that said, a nail will have less of a tendency to split a piece of wood when the air pressure is higher. Similarly, the nail will have less of a tendency to bend when it is moving faster too.

For this reason, I keep my 15 gauge nailer operating near its limit of 120 PSI, and the 18 gauge nailer near 80 PSI.

There is another benefit to this too!

Double Kiss
Many woodworkers have the problem of firing two nails when they pull the trigger a single time. This is called a Double Kiss. A double kiss is extremely dangerous since the second nail can strike the head of the first, and ricochet back and hit the user. It is also very bad for the driver and piston, as the second nail can impact the head of the first and the impact can jam the driver shaft and even bend it. 

The actual cause for double kissing is when the pressure-foot safety of the nailer is first released, and then reapplied. That is, the nailer will bounce off of the wood far enough for the safety to disengage, and as the nailer again makes contact with the wood, the safety is re-engaged; and a second nail will be ejected. So the actual cause is due to the bounce of the safety.

While the most obvious cause is that the user is not applying enough pressure to the nailer to keep the head on the surface, this is compounded when the air pressure to the nailer is too low. The low pressure will make the nailer bounce much more. (If you don't believe this, turn the pressure way down to say 20 PSI and shoot a nail. The nail will only penetrate the wood a little bit, but the nailer will lift away from the wood by a lot., no matter how hard you hold it down.)

Splitting Woodgrain
Splitting the woodgrain while using a pneumatic nailer is far less common than driving nails in by hand, but it does happen. The number one reason why the wood is split, is due to the wedge-shaped tip of the nail. As the nail penetrates the wood, the wedge will push the wood apart (cleave it) in order to make a hole. But if the wedge is penetrating perpendicular to the wood grain, it will cut its way through instead of pushing its way through. To really carry this to its fullest, a blunt-pointed nail will split far less regardless of its orientation. Some nail manufacturers will supply blunt-pointed nails just for this reason. Increasing the air pressure will also reduce splitting for the same reasons as listed above.