I've had my chromecast for about a week now. I really enjoy it. The ease of use is great, just select what I want to watch on netflix or google play movies on my phone and it shows up on my TV.
To start, its probably the easiest media extension for a TV. You plug it in, go to a website on your phone or wifi laptop, tell it your wifi password, and thats it.
Right now its functionality is somewhat limited. Only Netlfix, Google Play movies and music, and Chrome. But since it has chrome it allows you to stream (most) anything you are viewing a chrome browser (ie amazon instant video or prime).
There is a promising future though. Developers have shown it is able to stream movies off a network drive, off of dropbox, and directly off of your phone. While there are no other apps available Google did release a developer kit, so there will be more functionality in the future.
Here's to hoping it will get a native Amazon video app...not holding my breath though.