I'm currently using this sucker to frame my garage. I bought Hitachi plastic collated nails for it. It took me a bit to sort it out, but I was able to find 21 degree full round head nails at Lowes. I've been sticking with the Hitachi nails even though there are cheaper brands - the quality is good.
I used 0.131" diameter 21 degree plastic collated nails, 3 1/4" in length, for all of my framing. I was able to blast a bunch of 2x6's together in no time.
I then used 0.131" diameter 21 degree plastic collated nails, 2 3/4" in length, for all of my sheathing. The nailer handled both of these with ease. I can set the depth so I don't go splintering my OSB with too deep of a punch.
I am currently using the trigger that only allows one shot per pull. I like this feature as I don't go machine-gunning 3 nails on top of one another if I slip during the shot. Sometimes it is a little finicky, as you have to get the tip compressed just-so before you pull the trigger, or it will give you a disappointing "pphhh" and no nail. It also seems to have about a 50% ratio of actually firing the last nail in the clip. A lot of times it misses the last nail altogether. I did jam it once with the 2 3/4 nails because I inserted a partially used clip. The uneven break in the plastic collation misaligned the nail and the piston jammed the nail in the barell. I fixed it with a small screwdriver.
It's pretty light for its size. Some friendly helpers I had with the initial framing all remarked at the size of this thing. It looks big and beefy. You'll have a bit of trouble fitting it between studs that are 16" apart - it's a little bulky in tight spaces. For toe-nailing and mating studs together, I had to shoot at an angle. That's when the tip is most finicky, so you have to brace it on the back with one hand while you pull the trigger. At 90 psi or so it might not fully sink the nail on an angled shot like that. At 120 psi it sinks everything. It's kind of scary. My gun came with a protective rubber tip that is handy for things that you don't want to blemish, but without the rubber tip, the cleated gun is much easier to use for angled shots because the tip bites into the wood.
These problems are avoidable if you are careful. A nail gun is an amazing piece of machinery, as I am surprised that with the violence of each shot it doesn't go blowing apart after a few shots. Yet this thing will shoot and shoot as long as I pay attention to the clip, keep it oiled, and set it aside so I don't trip over it. I've dropped it off the ladder twice and it hasn't complained. It's also nice that you can direct the exhaust in multiple directions, as each shot will create a fine oily mist that you can feel on your arm, hand, leg, or whatever. That's just how it works with air tools. At least you can shoot it away from your face. Also, the rubber grip on the handle is slipping around slowly as I use it. That's kind of irritating, especially when it's hot, because the rubber will slide up and bunch up underneath the trigger. Eventually it will probably fall off with enough use. Could probably use a redesign.
On a final note, I bought this thing used from another online auction site that you can probably guess. It was well used before I got it and it's still kicking. For what I paid for it, I'm quite satisfied. I'd probably still be happy with it if I bought it new for a higher price.
Edit 9/6/07: I've been using the metal connector attachment. This thing is pretty handy if you are using Simpson Strong Ties or a similar metal connector for your framing. Most of the Simpson products require a 1.5" nail (so that it doesn't penetrate the other side of 2x lumber) and this gun won't go that small. I've used the 2 1/4" nails when I'm shooting into a junction or multiple pieces. It makes the work go much faster, otherwise you are stuck with a hammer. The tip for metal connectors has a triangular aligning piece that allows you to line up the nail with the hole in the connector. It works really well but be warned - the smaller the nail the better the chance that this thing will misfire. I had much more trouble with the 2 1/4" nails than the bigger framing nails. Also, I glanced off the connector more than once and shot a bent nail bouncing around all over the place - not like a bullet but enough to be dangerous. The metal connector work is much more sensitive to alignment so be careful. Also - be careful about lining up your shot, especially if you are using one hand to brace boards before you shoot. That nail doesn't care if your hand is on the other side of the board when you pull the trigger. I managed to nick my thumb pretty good at about nail number 2,327 during a 10 hour day. Not bad odds - but you'll be better off if you are constantly aware.
Edit 7/12/08: My F21PL is still going and my garage is pretty much done except for electrical and painting. I took this thing up in my attic to build a knee wall around the perimeter so that I could install foam backer board to keep insulation from spilling out onto my soffits. My attic is full of old fiberglass blown-in insulation, so it's nasty and dusty up there. I used my nailer for a few days and it stopped working. After messing with it I figured I had gotten dust inside and perhaps ruined a seal. I finished framing my knee wall with the Bostitch palm nailer (see other review on here - great little tool). I was able to download a rebuild kit from the Bostitch website instantaneously. There are also blow-up diagrams of all of their tools. I bought several rebuild kits and downloaded the diagram, and I was able to successfully rebuild the nailer and get it functioning again. I expect that with my rebuild kits I will keep this nailer going for the rest of my life. Great product support.