I have three. Two are jailbroken and have XBMC installed. One is "pure" and I've recently upgraded it to iOS5. From what I understand, you can jailbreak iOS5 on the ATV2, so I'll probably get them all upgraded/jailbroken soon.
I could make use of 1-3 more, but I'm still hoping to see 1080p output soon. Now that they've acknowledged 1080p via the iPhone 4S, I definitely think it will happen, but who knows when. That said, the other neat thing about this box is that it's so tiny and only requires two cables plugged into it, so you could easily get by with just one and move it from room to room if you wanted to.
For < $100 (and with no moving parts and no care about it looking pretty and scratchless, you may as well buy it as a refurb to save even more), it's definitely a great little box.