Unfortunately, it's complicated enough that it probably won't be extremely widespread-- players who haven't found the need to join up on voice chat yet probably still won't feel a need to do so. But for guilds who don't want to pay for a separate server and friends who group together often, the ingame voice chat should work just fine.
The rest of my impressions, including a complete walkthrough on how the chat system works, are after the jump.
The feature itself is easily set up, as long as you've got a microphone and drivers hooked up to your PC already (I only tested the chat on my Windows XP box, but Mac should work pretty much the same way). To set it up, you go into the newly retitled "Sound & Voice" menu from the options screen, where you're presented with two panes-- the old Sounds pane, and the new Voice pane.
Unfortunately, that tooltip covers up the two other meters down there, but those are Sounds and Music meters, so you can separately adjust those options as well, if you like.
Finally, you can see that the voice chat can be set as either Push-to-Talk or Voice Activated (ie, anytime you start talking, it will send your voice out). Personally I prefer Push-to-Talk, and I was glad to see that was the default option-- nothing bugs me more than hearing things I don't want to over voice chat. You can also check the box to have the game play a sound when you hit the Push-to-Talk button, and you can bind that to whatever button you want-- the default is the ` button, and that worked well for me.
So while it's easy to use if you get it working, getting it working is still not a super easy thing to do. And that, I think, will be enough to keep this from becoming a universal thing-- people in PUGs, whether they actually can't voice chat or whether they just choose not to, will still have an excuse for not having it set up. Guilds will be able to require it, I'm sure, but for makeshift groups and PUGs, I don't see this voice chat happening universally.
Strangely enough, even though the LookingForGroup channel is listed there, you can't join it by pressing the "Join" button-- you still have to actually flip yourself LFG to join that channel. But you can, for some reason, see who's in that channel without joining it. Not sure I understand the reasons for that, but maybe Blizzard is still working on it.
And creating a custom channel (you can see my "custom1" channel above) is as easy as hitting the "New" button. When you create a channel, you can name it, create a password for it (if you want), and you can choose to enable voice chat on it.
As far as I can tell, there is no way, however, to invite people to your custom channel from the Chat interface. So you still have to tell your friends to join the "nerfwarlocks" channel, and then they have to hit the "Join" button, and type the name in. It would have been really nice if you could right-click the custom channel, hit "Invite" and then chose the players you want to bring in, but there's no way to do that right now.
So what did I think overall? It's a good, robust system that will very likely keep a lot of midrange players from using separate programs for voice chatting. Super casual players who haven't jumped into a voice chat yet probably still won't feel the need to-- setting up the mic correctly (even before you get into the WoW client) is still going to be a barrier, and of course there's the social barriers of having your voice be heard by other players. If you haven't jumped that rung yet, the new voice chat system probably won't encourage you to do so.
And for bigger guilds (as in guilds with more than 40 players, like mine), my guess is that this system won't be enough. Sure, there's almost no reason to have more than 40 people in the same channel (and the Guild channel itself doesn't have voice chat), but one thing I like to do on Teamspeak is run around to the different groups and see how they're doing-- check in with the Karazhan group and see who they got for the Opera, or ask the Shadow Labs group how things are going in there with the new guildie. With this voice chat system, I can't chat with those groups at a glance, since you have to be in a group or raid to voice chat with the people in there. Not to mention that I can't tell you how many times someone's WoW client crashed on them during a raid, and we only knew what happened because they were still chatting with us on Teamspeak.
So Teamspeak and Ventrilo, in my estimation, have nothing to worry about. What's the point of this voice chat system, then? I think it will be huge for people who don't already have their own Teamspeak server, or for smaller guilds who don't want to pay for one. Good PUGs (as in, PUGs that consist of a few friends, or of a bunch of knowledgeable players from different guilds) will only get better, as people won't have to share their Vent info and coordinate all that madness. Some casual players may get more involved (and become better players), if they do make the jump to voice chat.
Players on both ends of the spectrum (super casual, and fairly hardcore) won't have a need for it. But at worst, WoW's new voice chat system will at least help midrange players coordinate their groups and strategies. And at best, just like Xbox Live, this voice chat system will be just another brick in the wall for bringing new types of social interaction into online gaming.