Our office firewall gets in the way of Yuuguu, probably by blocking UDP traffic I'm not sure what the real . Finally a coworker was WFH and we were able to try it. Hey, it worked. Lag wasn't bad, control wasn't hard to trade, and it didn't really get in the way.
It really shared the other guy's screen, not just certain windows. That was very convenient, but it also meant that I could see his IM and email notifications and the like. If you worry about such things, be warned. OTOH, it means that he could pop up editors, SQL tools, log tail programs, and the like without having to explicitely share them with me as he as to do in webex.
Yuuguu didn't include voice, but included a voice conference line. It does nice instant messaging and screen sharing, though. We skyped for voice, and yuuguu-ed for screen share. It can tie into your instant messaging accounts, which makes it easier to find people you know.
We had work to do, so we didn't spend so much time kicking the tires as we did trying to work through it. It was more important that it was transparent than feature-loaded. We didn't try five-way screen sharing (the max for the free version), nor the conference calling feature. I didn't see a way to do a shared whiteboard other than maybe firing up a graphics program on the host side of the talk. But that's enough most of the time. We don't use the webex annotation features that much either (too hard to get to them through the menu).
I give it a thumbs-up. If you can use it, give it a shot.