Use it combination with my NAS and it's brilliant. aTV Flash makes for easy connection and can browse files using the native interface, also plays pretty much everything (.mkv).
Airplay is pretty handy too.
I was pretty luck and managed to get mine through the refurb store when the ATV3 launched. I was tossing up whether to get the ATV2 or 3. I was SO glad I decided to get the ATV2 when I found out they hadn't been able to jailbreak the 3.
Personally wouldn't get one if jailbreak wasn't an option.