I'm a huge fan and have put in over 100 hours since release. I've been playing less in the past month in anticipation of the new update, but alas GTA came before the update. I'll be back in soon, though, I'm sure.
We definitely had a more active TF2 group on OO, but I don't know how many of them migrated to GT. If you're looking for a mature group to play with, the Gamers with Jobs group is a top notch bunch and their server is my favorite place to play ever since meeting them in the OO vs. Gamers with Jobs match.
Server? Are you talking PC?