I am building the website for a small dojo association and was looking for an extension to add custom fields to users profile when they register.(like their sensei name, dojo location, etc) I must say Community Builder made that pretty easy! I got the advanced subscription to support the team and get the whole documentation. After fiddling a bit more with CB, I found out it is a very flexible solution. But with flexibility often comes complexity, so I was glad to see that the documentation was very well written and complete. I found out about the privacy plugin feature for fields and how to add private tabs, etc. All in all, we managed to build very nice profiles for our users, and we are now looking at the other features like private messaging and allowing users to post their own blog and gallery.
What I also truly appreciated is that, while I am no programmer, I suspect CB API is really easy to use because of the amount of extensions that provide a plugin to integrate to CB profiles. CB itself is also pretty stable. I can't speak for 2.0 since it's been out for only a few weeks, but v1.9.1 ran without a hitch without problem.
And finally, the support. I got nothing but praise for it. While sometime I didn't like the answer(like: "This will be fixed in CB 2.0" which was months away back then) they have ALWAYS answered within 24 hours on their forums. The memberships price are not cheap(but you can get the basic extension itself for free), but I am glad I took one as a "thank you" for this great extension. It is unique in it's field right now, and v2.0 fixed the only small annoying things I had to say about it(the registration form and login box were not responsive so it would not display correctly on a tablet or phone. All fixed now). 5 stars