Just throwing in my two cents, for what it's worth (everyone else in this thread clearly knows a fuck of a lot more about what they're talking about than I do).
I have a Behringer US600. Works great in a feedback loop set-up in conjunction with some kind of distortion to produce HNW type textures, and generally offers a lot of options as far as pitch-shifting in general goes. One possible issue (depending on your subjective taste) is that, if you pitch-shift too high, you get really blatantly digital-sounding, squealy sounds. I like it in some contexts, but I can imagine that a lot of people would HATE it.
Also, I use tons of Behringer equipment, and I've never had an issue with any of it. Great if you're on a budget (like I am). If you've got the dough, then it would probably be wiser to buy higher-end equipment, but if your funds are limited, then at least consider Behringer. Just don't throw it around the room, and you should be fine.