Although I'm a fan of Ventrilo (ver 3.0.5 - 64-bit), I have found that World of Warcraft's built-in voice chat works great; once it's been set up correctly and executed. I preferred it over Ventrilo for several personal reasons, but that joy came to a screeching hault as of Patch 4.0! Blizzard's buggiest patch since I can remember (WoW junkie since Fall of 2005, <Bow, Bow>).
Having used both voice-chat programs for a period of time, I would like to offer up my own (humble) pros and cons list of both apps. I hope it helps someone/anyone out there, and perhaps stir up some conversation or honest debate about which one is truly the champ.
Ventrilo
Pros: Very customizable, loaded with features, and comes equipped with several codecs giving the user lots of versions of voice-over-game programming which allows for support for several sound card manufacturers and types (i.e. Built-in sound cards vs seperate sound cards for professional/advanced sound card enthusiasts).
Cons: Intimidating to the less-than-savvy computer users/gamers out there that go into Ventrilo's [Setup] and are immediately challenged by the dozens of tabs, buttons and selections that make Ventrilo so popular to elitist gamers out there!
World of Warcraft Voice Chat 
Pros: Simple setup and options making it a quick and painless setup. It sounds crisp, clear and uses less RAM then Ventrilo does (probably due to the fact that it's already built into WoW and not a separate entity). And due to the fact that it's built-in to the game, WoW Voice should, in theory, reduce the amount of programming conflicts and bandwidth problems that have been reported in Ventrilo. Lastly, users can create and edit the names of voice channels on the fly, and it's so easy to setup and use that my 9 year old son (at the time) was able to get it set up and working without any help from others.
Cons: Because of WoW Voice's simple setup, it doesn't have anywhere near the customizability that Ventrilo has, or the kind of support and popularity that Ventrilo gets. Additionally, Blizzard itself (and addon creators) haven't shown much support for its own creation since its release, even though Blizzard has acknowledged and responded to reports from players (me included) of an annoying bug that's existed since day 1 which kills the ability to hear and speak to others once you switch to another character without totally logging out of the game first.
On a side note, Blizzard has reported that they plan to have WoW Voice streamlined by the release of Cataclysm, offering up some bug fixes, customizability, and some real competition for Ventrilo and its faithful followers.
I'll get down from this box now. Anyone have any comments, complaints, questions, veggies/fruits to throw, etc? Thanks for your attention, all!
Osiris