I am a contractor and use a nail gun almost everyday, and I love my Hitachi framing gun. I would only buy a clipped head gun, you get more nails in the magazine each time you load it, as there are no gaps between the nails when they are in the clips. Also, the full-head clips are held together with plastic, and the plastic flys all over everytime you fire the gun, and gets all over the floor or wherever you are working, not to mention flying out at you and your eyes. (Always wear safety glasses no matter what pneumatic gun you are using.)
As far as brand, the Harbor Freight as mentioned may be a decent gun for occassional use, they sell one there that can be converted to use clipped or full-head nails.
Some guns will shoot up to 3 1/2 inch nails, but I rarely use longer than 3 inchers.
You can also buy a palm nailer which may serve your purpose fine. The palm nailer uses multiple hits to drive the nail, and is very compact, and uses regular bulk nails (non-collated) and can shoot different lengths, different diameters, and different types. I've used mine for everything from framing nails to roofing nails, and joist hanger nails. A palm nailer will even shoot huge pole barn nails.
Check out
www.toolsofthetrade.net, or amazon.com. Amazon has reviews by people who own or have used a tool telling how it has worked for them. Some of them are idiots, who obviously have no idea what they are talking about, but just take them with a grain of salt.
Also, like stated above, keep them oiled, and you will be okay. If used infrequently, I would oil before every use.
kevlars