Having taken the time to acquire a 2" J-frame and a 4" K-frame in .22, as well as a conversion kit for 1911's, I'm now looking at getting a setup for my AR-15.
.22 is not only a lot cheaper than centerfire ammo, it also allows a long gun like an AR to be fired on indoor ranges and at other caliber-restricted venues.
Having the .22LR understudies allows me to get in a ton more trigger time than I would if I was limited to their full-bore counterparts.
Michael Bane has some interesting thoughts on rimfire practice. Definitely worth the read...