Having recently sold my Apple TV, I would make sure you really need one first.
If you have cable or satellite hooked up to your main TV, then there isn't much for the Apple TV to do other than be a place to showcase photos and videos from your iPhone or iPad, or play a game, if that's your thing.
It basically hooks up to the various cable channels via apps, so you can watch those networks live or on demand. Now, if you have satellite or cable, your provider already has a lot of that available through your set top box.
If you have a TV in another room, you can save money by getting rid of the set top box on that TV and hooking up an Apple TV to it, and use the various channel apps to make the TV an on demand experience.
Personally, I only have one television, so I felt the Apple TV to be a bit redundant, although I enjoyed the picture viewing capabilities for when family visited.