Let's start with the good: Google's Chromecast is the best way to watch Youtube movies on a TV. You can start up that Metroid speedrun or a Maru video and relax, comfortable in the knowledge that you're not burning cycles decompressing video while you're compiling the latest version of your new app. It's also great for listening to music you've uploaded to Google play. If you subscribe to their "All Access" service it's even better as everything they offer is seamlessly integrated into your personal library. Being able to browse your music library (once you've finished uploading it all to Google's servers) on a desktop before initiating a stream directly to the Chromecast is a great feature and one that Apple should steal. On top of all this it's cheap (only $35) and it comes with all the cables you need in an attractive and well designed package.
That is, of course, assuming you have a computer or smart mobile device (Android or iOS, sorry Windows Phone) as you'll one need one to complete the setup process. It's well done but much like the device itself, it's not complicated, but unusual to a point that it's borderline unintuitive. The attractive little HDMI dongle also loses much of it's appeal when you plug it into the long USB cable it relies on for power.
By far the worst part of the Chromecast is also what makes it unique. The device is basically a "headless" web browser with no UI of it's own. You'll find yourself scrambling to find the tab you started that Youtube video from (if you haven't closed it already) when you need to pause the stream to go answer the door or "nature's call". The Chromecast extension (for the Chrome browser natch) functions acceptably well as a "remote" for the device but I always forget it's there and grabbing the Apple TV remote off my desk is still much more intuitive.
Google says that Chrome "tab casting" is in beta and it shows. Apple's AirPlay technology is incredibly powerful and solid and everything tab casting is not. It only works with Apple devices but if you're reading this site you're probably mirroring this page onto your 60" LCD TV (I hope not).
Bottom line: If you have a spare HDMI port and somewhere to plug this thing in for power (USB or AC) it's probably worth the cost of a nice bottle of booze (can I recommend one from my home state?). The fact that Google lets you upload 20k tracks for free to it's servers (ala iTunes Match) just sweetens the deal.