I have both the PS3 and the ATV2 and I enjoy both but I only use the PS3 now for playing Blu Ray discs and PS3 games. The PS3 isn't energy efficient enough, cool enough, or quiet enough to just leave on 24/7 like the ATV2. And it doesn't have that "instant on" ability iOS devices like the new Apple TV have, so we prefer the Apple TV for regular use.
The major advantages of the Apple TV over the PS3 is the better UI and the nice integration the ATV has with your iTunes library, iPhoto or Aperture, Mobileme, and all of your iOS devices. The ability of Airplay to send music and video from your music and video playing iOS apps to your TV and home stereo is another benefit you can't get with the PS3. My family all owns iOS devices so it was a no brainer for us. The price was low enough that we could put one at all 3 TVs in the house to have all of our content available no matter where we are. I couldn't do that with the other solutions out there as it would be far too expensive. And the Wife Approval Factor is high with this little box.😀
If you don't have any iOS devices and you don't or don't want to use iTunes to manage your music or video library, then if you have a PS3 already an ATV is not necessary. But then none of these devices are really "necessary" anyway. 😛