On my HTPC I'm using it purely to watch Youtube video and other video hosting sites as well as the streamed media from my main HTPC in the living room.
What I hear about jailbroken ATV2 that really got me thinking is the capability of watching cable contents (can someone confirm this?)
I have XMBC on the laptop in the BR but only use it as a front end to watch the media on my LAN. Everything else I just go to the browser and watch it from there.
I'm cable bill with HBO and Showtime is getting expensive, if I can watch HBO/Showtime and other premium stations via the ATV2 then I think it's a winner.