I love how Leicas work, I think the Leica M3 is about as nice as cameras get. Incredibly simple, beautiful to (be)hold, and I find range finder focusing so much easier than the SLR split screen.
However, Leicas of course cost a lot, and don't have the accurate framing of an SLR. An SLR setup comparable to a Leica M3 can be had for 10% of the cost, and they really are just as good. Leica lenses are great, and nice to use, but they're not unbeatable. I've shot Leica and other glass, I can't say the best lenses were Leica.
Leicas are outstanding, no question, but as a general rule, SLRs are probably more useful. Having said that, I'm a sucker for a beautiful camera, so if I wanted a nice 35mm camera, I'd be getting a Leica. You can keep prices down with a screw mount Leica, or perhaps a Bessa or Zeiss Ikon range finder, the Ikon in particular is very handsome, and very functional.