I just joined OSC's online writer's workshop (that's Orson Scott Card, folks) for several reasons:
1) the users provide excellent feedback on writing
2) the users seem dedicated and consistent
3) I love OSC (or, more accurately, Ender Wiggin)
4) it isn't completely flooded with users, like most online workshops/forums
Plus, the experts encourage it.
It's actually a pretty cool setup, not too fancy, not too ambitious. The users are friendly, honest, and actually give feedback (so hard to come by on other sites, despite what they advertise!)