I've seen the AppleTV, but I've put off buying one until it supports more video formats. I think the biggest issue I have with it is that it really seems designed to force you to buy more stuff from the Apple store instead of being able to work the files you already own straight into it.
The GUI on it is really nice, as is typical for Apple.
Yet, the PS3 does a fair bit more, for not a lot more, and offers some impressive upgrade paths which AppleTV does not.