I bought one, and it can really help if you've got weak pres (for example, it helps out with my MR-8). It also does some pretty cool things with instruments, like if you use it instead of a bass DI. I've read that it even can fatten a guitar if put in front of a digital modeller, though I haven't tried it.
Don't think, though, that it will "make better vocal recordings" for you instantly. It just changes the sound in a way that may or may not please you or your mix. As far as the change goes, it seems to fatten things up and "warm" them a bit, but overuse certainly will make things muddy.
Definitely worth fifty bucks. However, be reminded that it is a fifty-dollar tube preamp. Some people pay hundreds just for tubes.
The build quality is okay, but it's something that someday (sooner or later) will benefit from a re-tube, a refurbishing, or a full replacement.