I just got accepted for the joost beta... and I can confidently say that it's not what I was expecting. The interface feels slow and clunky, and it's not very intuitive at all.
The content is relatively old, 2005ish for a lot of things, and most of it is simply not that interesting. The National Geographic channel is alright, but the shows in there are even older than most of the other stuff on Joost. It's ok I guess, I guess I was just hoping for more recent NG material.
The guinness records channel is boring and dry, I thought it would be like the fox show that they had several years back, instead it's some british knockoff, and frankly, the record attempts are boring as hell. The Comedy Central Channel would be 10x better if they put some popular content on it, like south park, reno 911, and whatever else people watch on CC.
Joost has some potential, but the interface needs some major work, and the content needs to be updated regularly.