I recently held an exploratory meeting with a potential new client where we needed to share a screen over the internet. Neither of us had used DimDim but it did the trick with no snags.
The only thing that could have been considered a snag was the fact that I was using Google Chrome as a browser, but it quickly detected this and told me it didn't support it; FireFox worked fine. The heavy tie to the browser suggests to me that they are doing relatively specific and clever coding in order to make the experience relatively seamless.
Over the years I've tried numerous different screen-sharing and web-presentation solutions, and most of them have required some kind of adjustment in order for them to work decently. Usually you need to reduce your screen resolution in order to get decent performance, but DimDim didn't seem to need this. The responsiveness seemed quite tolerable.
In addition, you don't need to install anything, which reduces the reaction time -- you can more quickly decide to share screens and be up and running. Because DimDim provides a free 20-person webinar membership, and a very reasonable 228$/year for 50 people, 75$/month for up to 1000 people, you can try it out without worrying about the cost of scaling up (actually, I think you can do a lot with just the 20-person free version).
One thing we didn't try was voice; we just talked on the phone. If you were doing a webcast-style webinar you'd need to broadcast voice as well.
They are clearly trying to eliminate the costs and difficulties of web meetings and webinars, and as far as I can see have done an admirable job. I think it could open up more possibilities; I will certainly be trying to imagine ways to make use of this new resource (not "new concept" but "new" in that it is a seamless implementation; the effortlessness is what opens up the possibilities).
I was disappointed with dimdim. I was truly hoping for the type of experience you had as I'd like to use the tool with clients who won't have the patience or technical knowledge to deal with all the problems that arise trying to get stuff like webex to work.
I spent about fifteen minutes trying to share my screen with a friend and couldn't get it to work. I suppose if I spent more time with it I could have figured it out, but the whole point is to find a service that "just works".
But I know Bruce has a pretty low tolerance for brokenness, so maybe I'll give it another try :)