The combination of Apple's polished user interface, and the AwkwardTV additions make the Apple TV a killer system for me. It's fun to us, can handle more of my multimedia library than any other network multimedia system I've tried so far, and is easily extendible.
The only downsides for me at the moment are its lack of 1080 support and its reliance on third-party developers for full functionality. But even if nothing more ever came out of the independent developers, the system would still be more capable than anything else I've tried so far.
Some might argue that deploying a PC with Microsoft's Media Center is a better answer. But I don't want a full-fledged, costly PC with its excessive noise and power consumption; not to mention having to reboot the TV every "Patch Tuesday" that comes along. The Apple TV is tiny, nearly silent, draws only 18 Watts of power and starts at $300. For me, that's a hard combination to beat.