I've had an iRobot Roomba 530 for a few months now. Here's a video of it in action in case you've never seen one:
That's not my video, but that's pretty much the same kind that I have.
When I first got it, I sat around and watched it in action for a couple of days to see where it would potentially get stuck and where I'd need to put up the virtual walls. Then the newness of it wore off and I'd just ignore it and let it do its thing.
I really love it a lot and kind of wish I had two of them. One for upstairs and one for downstairs so that I wouldn't have to bring it up and down. At least the Roomba is a lot easier to carry up and down stairs compared to a normal vacuum, especially compared to our old Fantom.
It is great at picking up all of my long hair. Too great, in fact. I end up cleaning the rollers and emptying it after every use - the Roomba manual recommends after every three cleanings, but for our house, it's necessary to clean it more often. After about every three cleanings, I'll spray the filter and collection bin with a can of compressed air just to really get it clean.
It's pretty awesome that it has stair detection. So it's nice not having to worry about it going down the stairs when it's cleaning upstairs. I also love how it docks by itself when the battery is low or when it's done cleaning a room. And I love the sound effects. It'd be kind of neat if I could hack the sound box and load some custom sounds onto it. :P
The best part about having a Roomba is being able to have it vacuum while I go and play Rock Band or just do something else in general.
One kind of annoying thing is if you have a cluttered floor, you have to block it or move all of your stuff first or else the Roomba will scatter your stuff all over the place.
I kind of wish they were smarter about knowing where they've been before though because sometimes it'll just go in zig-zags and end up stuck in one general area.
After having one, I don't think I could ever go back to using an old vacuum again. I'd definitely get another Roomba because they're worth it. Sure, it takes longer to get your vacuuming done, but at least you're not the one spending the time to do the cleaning.
I thought this was a pretty fun idea for a Roomba hack:
Cool! I want a Pacmba!