I have a Walther P22, and it is surprisingly accurate and reliable--with the right ammo, an when it is is clean. It gets progressively less accurate after 100 rounds though, and can be exceedingly finicky about ammo...
The Ruger MkIII is one of the finest semi-auto .22s out there, especially the ones with bull barrels (for balance and "feel"). The idea of a .22 semi-auto pistol that is ALL steel is VERY attractive, and my next .22 will without a doubt be a bull-barrel MkIII. They are super-reliable, will cycle just about any ammo you put through them, have pretty good triggers, and are laser-accurate.
My only complaint about the Ruger MkIII is the capacity. Those stupid 10 round mags they have are ridiculous--they should be 15 or 20 round mags. They are super-fun to shoot, and ammo is dirt cheap, and having to stop and reload every 10 shots is annoying as heck. (but it does slow down the speed with which you can burn through a brick of .22 ammo...
)
Not a big fan of the polymer frame .22s (except the Walther). Most of them feel like toys to me, and I like a gun with soem heft to it--even in a .22...