There are actually four voice solutions of which I'm aware: Vent, WoW Voice, Teamspeak, and Mumble. Of the four, I've found Mumble to be most reliable, clear and easy to set up. My guild (Soulbound, Inc., Perenolde-US) switched from Vent to Mumble some time ago, and we haven't looked back.
That said, I think that if Blizz gets a few more kinks out of WoW Voice, it would be a good way to go, too.
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