I just found an invite the Joost beta test in my inbox. To summarize the experience in one word: AWESOME!
The user interface concept is just what you expect from a state-of-the-art service. Transparent overlay menues are nothing new, but it is good to see them used well. The channel navigation is great. The viewing experience in full screen is nice. Although most footage is not in HD.
I just got 16mbps DSL service from the pink giant (Deutsche Telekom), still I got several hick-ups watching footage.
The biggest issue with the current Joost service i found is that fast forward is not working. Apparently it should work to move the slider on footage that I already viewed, but that did not work either.
Fast forward on something I did not watch before is not supported right now, based on a comment in the Joost forum. Fast forward is an important feature. I hope to see this working in the next version of Joost.
There are also already short commercials featuring Eclipse and T-Mobile. They appear switching between shows. The Skype founders Niklas Zennström and Janus Friis announced that their new venture will be called Joost in January. In February they announced their first important content deal with Viacom.
You can apply for closed beta of Joost on their site.