I bought my brand new iroomba robot hoover in 2012 for £200 delivered on amazon. It was an older model, which meant there no timer (I have to be there to press 'start' and 'stop' each time) and there were no invisible walls you could set. So I have to close a door if I want to restrict where the roomba goes. But that's a small price to pay as my downstairs is all one level and quite open-plan (but with doors they can close too). I love that I can just press start, go out, and find the roomba is nearly docked after cleaning my house beautifully!
What it doesn't do it whip round quickly like you can with a manual hoover. I looked at a YouTube clip showing green light for the direction of travel on a robot hoover, and it was so random! The whole room got cleaned but not the most logical way, and certainly gives the impression that it's missing bits. The YouTube clip showed it completing a room which looked like it missed bits, then repeated it exactly but this time showing the green light of the path it had taken, and by the end the whole room was green which proved it covered every inch.
You can't do the stairs but it stops automatically from falling downstairs, fascinating to watch! You have to clear the floor or it either sucks it up (Lego) gets stuck on it (sheepskin rug) or bumps gentle into it (chair leg) and turns around to avoid it next time. It fits under a lot of furniture though, so I often let it rub round chair legs as it eventually squeezes in between and clears under the table. It in a helpful mood I'll maybe move one of two chairs to the side to make the gap bigger to save time. Sometimes if I can't be arsed to clear the whole room I will lie a chair down to block the lego off, and just do one area. The other option is to press 'spot' clean, which has the roomba do a few circles in one smallish area only, rather than freely roam the whole room.
You can't hoover a wet floor but I think most hovers are like that? I gather you can get some that moo for you now as well, but haven't seen any reviews and think you pay double the price. The empty mechanism is much more easy and efficient than my dyson cordless (which I use for stairs, car, if I can't be arsed to pick up all the toys etc).
Hope that helps!