I really like the idea of Mumble. The only problem is that the best feature (positional audio) does not work with BF3, but that may change in the future.
But there is another problem that weights even more (for me). I liked the squad talk feature of BF2, sadly it was removed in the PC edition of BF3. Mumble or even Teamspeak is a perfect replacement for it, but only if you are playing with a couple of friends (where you also can use the Party feature of BF3). The problem is that most people don’t join a server or if they join, it is hardly the same you are on. Another problem is that someone has to manage the channels to represent the actual game (set up channels per team and so on). This makes it too hard to simply use it with a couple of unknown people on a server.
Sadly BF3 looses a lot of its tactical gameplay without any team communication. Hope that there might be a solution some day.