Been playing around with my Chromecast for the last few hours, here's my thoughts so far.
Nice device, and has a lot of potential once there are some more robust apps compatible with it, but right now it is definitely not a replacement for a Roku or Boxee-type device.
It plays Youtube, Google Music, Google Movies, and Netflix great, they all work very well, but those are the only 4 apps so far.
Playing "downloaded" video through the device from your home network is very sketchy. If the video type doesn't play in your desktop google chrome browser, then it obviously won't stream to the Chromecast. Right now, Google Chrome is not compatible with .avi files, and any .mkv files that contain AC3 audio(which out of my hundreds of HD movies I have downloaded, is a LOT, I'd guess maybe 50% won't play with audio in Chrome)
Using it to project your PCs web browser to your TV is nice for static images(pictures or documents), but the screen is VERY laggy if you are scrolling around a lot through a large document or webpage. I'm sure some of that has to do with your wireless connection, but I was testing in a pretty typical situation with my ASUS wireless-N router in the same room as my laptop and TV/Chromecast, that should be pretty close to as good as it gets with home use. Any web page that has video but does it through Silverlight, won't work on the Chromecast(namely, Hulu). The video has to be Flash/HTML5 right now to work.
Hopefully some updates(either first party or some creative 3rd parties) can get some more robust features and codecs into this thing.
So basically, if there's a specific app, it works great, sending your own files and web pages is REALLY hit or miss at the moment. I'd estimate that less than half of my video files worked. .avi is straight out non-functional and a lot of .mkv will give you audio or video issues depending on the exact encoding types.
Oh also, mobile Chrome browser (phones/tablets) has no Chromecast plugin, you cant send browser pages from mobile devices, they can ONLY use the specific apps that are built for Chromecast so far.
Once the Plex app is available for it, that should make a world of difference. I hope it's out pretty soon.