Well I'm very happy from what I am seeing here, video quality is high, no problem with 1080p content. I am seeing a little frame rate/sync stagger on my 1080p monitor here at work, but very very minor, it's certainly good enough for general convenient viewing and throwing content quickly to the TV.
Using chrome I easily streamed HD content using the tab stream feature from
Hulu.com
Youtube.com
Vimeo.com
ESPN3.com
As others have posted, Silverlight and Quicktime sites don't work with full screen video
Tried the apps that support Chromecast on my phone, namely YouTube and Netflix. They worked well, especially Netflix who's interface is quite nice.
So, for an out of the gate new product I think it's very impressive at the price point. I wouldn't use it to replace my hard-lined Roku box as it doesn't have the full support yet, but hopefully devs will continue to build it into their media offerings. Looking forward to seeing it included in Plex as I use that a lot both at home and on the road to play my local content/movie archive.