I received my Joost invitation a few days ago. I have a few quick thoughts I would like to share:

• It takes a really long time for the Mac client to start.
• Once it finally starts, the overlay interface is pretty neat.
• The first day I used it, things wouldn't stream properly at all and I figured the Joost servers were getting slammed. Today, working beautifully. Not a single streaming problem. Hopefully it was my connection and not Joost.
• Obviously, they need more content. Hopefully when more users sign up and get behind this great service, more content owners will get on board.
• I am totally okay with ad-supported on-demand television. I am behind it 100% and really hope their content continues to grow. (I've only seen a single, two-seccond, ad so far though. Where are they?)
• Since I don't know anyone who uses Joost, I haven't been able to really try out the social features. They look pretty neat though.