I just got an invite to it yesterday, and I'm checking it out right now. Looks pretty sweet.. You get an invite via e-mail, download the program via a link, make an account, and then you can choose from different channels to watch.. There's not a toon of content yet, but more than I thought. Lots of WPT

The program will be interrupted now and then with a short commercial, but it's worth it.
If you wanna know more, you might want to check out
http://www.joost.com/whatsjoost.html ..
Edit: Right, not the most stable of apps, but seeing as it's still in beta, that's kinda understandable.