I can give you some information first hand and pass along some anecdotal. I have the SN65. Admittedly I've only driven a few nails with it, but it seems to be an impressive tool from the company that seems to manufacture the industry standard. (I also have the SFN40 finish nailer, which I like, as well.) I read one post somewhere from someone (allegedly a contractor) who had a Senco, didn't like it for various reasons and switched to the Stanley. I was a little suspicious of the post; it seemed like he had an axe to grind.
When I was first looking, I decided I wanted a full round head nailer instead of the clipped head. As it turns out there seem to be several codes that are prohibiting the use of clipped head nails (anecdotal). I was somewhat confused by the various nomenclature of the Senco products, so I called them up and got the following:
SN325 clipped head (compact nailer, been around for a while)
SN70 clipped head
SN60 round head
SN65 round head (The SN65 has a more powerful motor than the SN60)
SN65C round head, shoots .162 nails instead of standard .131
The .162 nail is apparently required in California.
I was told (by the Senco lady; they are very nice on the phone, by the way) that the .131 nails don't play well in the sandbox with the SN65C, so it's kind of "when in CA, get the C, but otherwise..."
According to their website (http://www.senco.com), the new ProNail replaces the old clipped head series, but is itself a clipped head with a larger head to give the same area of a full round head.
Clipped head nails (and ProNails) strips can hold more nails for a given length than the full round head, thus from a productivity point of view, they are more efficient. Full round heads are also more expensive because of the collating required to keep them in a strip. Picture a machine gun belt; thats what a full round head strip looks like. A clipped head strip has the nails abutting each other directly.
I have heard that Senco nailers sometimes cough on non-Senco nails (anecdotal) but the few non-Senco (All Spec made by Stanley) that I have used worked just fine. I am always suspicious of those stories, assuming they are a marketing ploy. Senco nails are harder to find than the All Spec (Home Depot has some All Spec), and may be more expensive (anecdotal).
If you are looking to buy a framer to use on the job, talk to some Senco users. They don't stay in business and get the prices they do by producing junk. But, like routers, there are people that are fiercely loyal to and against them.
If you're looking to buy a framer for your occassional odd job, it's probably more gun than you need in terms of cost. Porter Cable makes a less expensive alternative that, in light of their other tool's performance, should do well for you.
The Stanley-Bostich SN80 is the second industry standard, although it is a clipped head nailer. However, in a quick look at their website, I didn't find it, so I wonder if they have phased it out in the face of the more restrictive building codes.
The Hitachi nailer also gets good marks in the reviews, and may be the 3rd industry standard. I believe they make a full round head nailer.
Be aware that the Senco comes with the so-called "bounce" or "sequential trip" trigger standard, which in my opinion is dangerous for the odd jobsman, but amazingly productive for the pro or experienced amateur. They will provide the restrictive trigger at no cost, and it is an easy, no-tool, field change to install it. Changing back is just as easy.
Any compressor that can give you 100 psi should provide enough air flow to run one. A 3/4 hp 8 gal tank model would be plenty.
Have fun; working with air is neat. It makes you feel like a real pro, even if you're hacking up the place.