:D I have to agree mumble do come up trumps on a good 90% of comparable attributes and only really falls on its limited and clunky admin options/utilities, that said their dev notes suggest that those will be stamped out in the next couple of updates.

As for my non willing to convert (dispite already having it installed) is 90% of peeps I play with use Vent ans TS.

I will say no more on the matter ;)

My reasons for using are as follows;
It is faster and more secure than Ventrilo or TeamSpeak and offers better voice quality.
Mumble was designed for gaming!
Low-latency, high-quality voice communication.
Mumble is open source.
Mumble utilizes positional audio, voices come from the direction of in-game characters!
Mumble uses Speex not only as a voice codec, but for real-time echo cancellation / noise dampening

I might use mumble if I really have to, for the record my preference's in order are:
1 - TeamSpeak
2 - Ventrilo
3 - Steam Chat
4 - Skype
5 - Mumble

:p