I joined about 10 months ago to get info regarding my first ever "protest" (the Security and Prosperity Partnership, or SPP).
After joining I started to get a whack of e-mails telling me that 'so-and-so has added you as a friend on facebook.'
Turned out to be tons of friends--mostly out-of-town--who I hadn't been in touch with for ages.
In that sense it's pretty cool.
The available groups, on the other hand, seem to be pretty lame from my limited experience.
Worth it just for the former.