I use Boxee Beta on my ATV and love it. Since I don't have any iTunes video content I use the Boxee app exclusively. I enabled USB disk storage and afp access, so I keep all my media on a 1.5TB hard drive attached to the machine.
Since the hardware will likely never be upgraded, I'll move to a Mac Mini once it gets the audio over Mini DisplayPort so I can play 1080p content. Until then, I'll continue to enjoy my ATV.
Ive been using boxee with my MLB.Tv sub and the one thing I love about there MLB app Boxee has is that there's a option to hide the scores so i can chose if I'm coming home from work watch the game if its still going on or start from the beginning with out being spoiled from logging into MLB.com first. normally and the quality has been better than the website so far.