It's just another box to stream content, really, like Boxee, Roku, Google TV.
I suppose the advantage of having an Apple TV or any other streaming device for that matter, is in the rollout of services in the (near) future. For instance, Apple will announce the iCloud in a few days.
If you have a PS3 there's no need for an Apple TV. Most other devices like the Apple TV have the added benefit of plugging a USB drive (or streaming from a network) and playing almost any video or audio file. Apple doesn't have that so it's worthless if you already have a device that streams video content unless you buy a lot of stuff on iTunes. If I was going to buy a device only for streaming purposes I'd get a Logitech Revue. Personally, I have a PS3 which I use for streaming and a WDTV Live Plus which I use for video files.