I tried using mumble ages ago when it first came out when a mate of mine wanted to switch to it for the latency benefits (we were people playing a game from all over the world - Europe, America, South America etc) but nobody would really put into it.
The trouble is, most people are just kind of stubborn. When I get a CS mix people just ask "vent?" because they have it installed. To break a whole scene out of a habit might take a bit :P
Mumble overall delivers the best performance imo but it's not so user friendly. We @ lvl^ use mumble and many ppl when need to come on board for a cast or whatever have trouble setting it up properly.
Best thing about mumble is the auto voice volume adjustment aswell. I've always found the need to manually set peoples volume in both ventrilo and teamspeak 3 more annoying than 5 minutes of help in actually making mumble run for certain individuals.