We've tried Oovoo, but we gave up because of frequent dropped connections. The average connection length was as little as 10 minutes.
Oovoo is also not free for the resolution we needed (640x480 15fps allows both whiteboard reading and facial expression interpretation) and for multi-user conferencing after the trial period. I'd have argued for payment if it had been more reliable.
Lately we've been using Google Video Chat (Gmail) point to point, and doing limited multipoint with multiple computers. We'd like a reliable multipoint solution but haven't found one at the resolution and edge sharpness we need.
GVC stability is very good on XP. Unfortunately the OS X client is unstable, we can't maintain an XP to OS X session for more than about 10-20 minutes.
We're using Logitech Vision Pro webcams for their build quality, optics, and driverless operation (they're marketed for OS X, but are driverless on XP SP2 and Vista as well).
We're also using Panasonic IP Cameras with embedded web server to serve a 640x480 JPEG refreshed q3sec to up to 12 remote clients, but that's only useful for sharing diagrams and static material.