I don't tend to join many, and half of the ones that I join, I do everything as part of my 'normal' regimen (like BootCamp), so it's no biggie.
The main one I'm in right now is Ovation. Shoulderlength in 08 is gonna happen as a result of BootCamp. I'm 'doing' Cardio for Hair, but I haven't posted in the thread, yet. *thinks* The AAA Challenge is just part of my reggie now, too.
Challenges have become my way of trying something out to see if it works in my reggie/my hair.
They are usually long enough to see some results, but limited enough that I don't feel like I'm going super overboard.