I grabbed an invite to the Joost beta test last night, and gave it a shot (Joost, if you remember, is the peer to peer -P2P - streaming video application from the billionaire makers of Skype and Kazaa). First impressions are pretty good - the interface is smooth, changing content happens quickly, and video loads almost immediately. The way navigation overlays above the video content playing at all times is pretty cool. Also, the widget overlay was a surprise to me, and very cool - you can add RSS feeds, Google Chat, group chat rooms about the video you're watching, and more. When you IM with someone via the Google Chat widget, you can see right through every window and continue to watch content.
The value of this is of course going to be the content - what's available to watch. To be big, Joost needs to offer some A-list content, not shows that are less popular, or discontinued, or niche channels that won't appeal across the board. There are some major advertisers on board, so hopefully we'll start seeing sponsored content soon. The advertising spots are expected to be a mix of pre-mid-post roll, ticker ads on the bottom, and branded call outs in the corners, similar to broadcast TV. Say what you want about everything moving online - a desktop video application that offers internet features is easy to use, and could easily take off if the content's right.
If you want an invite to Joost, e-mail me - I owe Mashable for giving me my invite, and I'll do my best to send some out myself.