I've owned a P22 for three years - it was my first gun.
It's a terrifically fun, utterly ergonomic pistol that just feels right in your hand. I love it for introducing new people. Its internals are a smidge more complicated than they probably need to be.
I got mine for 279 bucks. Now, they go for 300 new. For 279, I'd say they're great. For 300... it's no longer a Definite Buy, now just a pretty decent buy.
The initial run had problems, but it looks like Walther worked out the bugs by the time I got mine. I'd definitely say to go new over used, if only to guaranty that you won't get stuck with one of the first flight.
Be aware that if you get one, you should break it in for the first 500-1000 rounds with CCI mini-mags. That's what I did, and it then ate bulk pack ammo pretty well.
Long term?
Well, in the last 1000 rounds, I've started getting FTE's. I'm not that disquieted by that, mainly because it's been 3 years and close to 20k rounds before they started popping up. I called S&W, and they said to send my pistol in.
Also, if you decide to start collecting stamps, the threaded barrel offers lots of tasty options. (Provided you're not in CA.)
So, final analysis: Used to be a DANG GOOD value, now just a decent value. Good plinker. Treat it well, and you should have a fairly darn decent pistol that S&W/Walther will stand behind. My P22 always goes with me to the range - in fact, that's why I haven't immediately fedexed it to S&W - I have too much shooting to do before my local range closes for renovations this weekend.
If you want a dedicated target pistol, you might want to take a step up. If what I've described is what you're looking for, then you'll like it.
My single slug's worth of lead