To whom it may concern: I just got my Chromecast in the mail today. I had a few headaches getting it set up, but now that I have everything working, I think it's going to be awesome. I'll do my best to help you avoid the problems I ran into.
Google is definitely going for an "automagical" experience which means that everything is just supposed to work. I'd guess that it does for most people, but it didn't for me, and they provide virtually no information about why. In my case, the problem was that I tried setting up the device with my desktop which was connected to my network via ethernet instead of wifi. Google didn't mention at any point that it needed to be using wifi, but the setup won't work over ethernet. This makes sense now that I understand how it works, but I was very frustrated at first.
The main reason that I'm excited for the Chromecast is so that I can watch video from websites that don't have apps on Roku or XBox. Unfortunately, when I first tried casting a tab to my tv, the flash video wouldn't play in fullscreen mode and the audio didn't work. Google's support docs say that third party tools such as Silverlight and Quicktime won't work, so I assumed that Flash also wouldn't work. It turns out that Flash works fine, but only if it's the special "Pepper flash" that comes with Chrome. I've had problems getting pepper flash to handle the DRM used by Hulu and Amazon in the past, so I had it disabled in favor of normal Flash. If you're having the same issue, just go to chrome://plugins/ and switch back to pepper flash. Once I did that, flash video worked perfectly on my tv. The video is crisp and not choppy, and the audio is perfectly in sync.
The Chrome extension has a few settings that I tried playing around with. When audio wasn't working, I turned on "Audio mode" thinking that might help. Then when audio did start working, the video was really choppy. Turning off audio mode fixed that. I have no idea what Audio mode is supposed to do, but I'd recommend staying away from it.
I considered buying two Chromecasts, but I was worried that having two on one network might cause problems (Google has a history of being horrible at handling multiple accounts). From the looks of it, they totally nailed it, and it should be really easy to manage as many Chromecast devices as you want.
Some content still doesn't work
I tried casting a video from Amazon to the tv, and the Chrome extension showed an error saying that the content isn't supported. That probably means that Google is checking some sort of licensing or DRM setting and not just blindly casting whatever I want. That's fine for me since Amazon has a Roku app, but hopefully this doesn't cause a bunch of misguided content sites to block the Chromecast.
This thing is awesome. The tab casting is easier and smoother than I expected, and the UI for switching between devices is really intuitive. This will make so many things easier including: 1) watching random videos at home, 2) having an easy way to watch video when you travel, 3) allowing multiple people to display content from their computer on a tv in a conference room setting, and 4) hooking up to a project or other presentation system. I've only had mine for a few hours, but so far I'm very impressed.