I used to be in a clan [EVO] had a server, was great fun got a bit bored with it after a while haven't checked recently but im pretty sure theres still a lot of servers and lots of people playing, albeit not as many servers as COD but theres more people per map
I have played Arma 2 and I would guess the official PR missions and game modes will have good stability. ArmA 2 have a realy big lack of Squad VOIP and teamwork. I think it should be implented a better squad capability better voip and better squad control.