There’s a recent thread on the WoW forums which addresses the ramifications of the voice roll out, online here

To sum up the thread – most (presumably hearing) people who were using Ventrillo before, have stuck with it because they find that it works better. Others complain that the in-game voice feature doesnt work (it is too quiet, voices get mangled, sounds are poorly differentiated, causes lag). Ah, the irony.

We started new alts the other week, now in mid 20’s, and it seems as if the voice feature has not made any difference to my experience of WoW (either with the new characters, or our mains). Nobody, for instance, has asked me about using voice in any casual groups, instances or in battlefields. As it happens, P’s guild still don’t use ventrillo or in-game voice, perhaps because they are RP (and his main is lvl 70 now).