I tried it once a couple of weeks ago, it was surprisingly good. I'd say a little better than NetMeeting at its default settings (you can tweak NM a little). Personally, I was wearing Sony MDR-V6 and my buddy and I both used some cheap computer mics. It's kinda tinny sounding.
Note that you become like part of a "super-node" for it to work.
If I remember correctly, this doesn't include spyware (I'm pretty sure the website shouts about that). They also have a PPC version, which I installed but haven't had a chance to try yet...thanks for reminding me.