I gave my wife the Roomba for Christmas. We were sick of picking up the dog hair in the front hall (ceramic tile) and off the medium-pile carpet everywhere.
It works VERY well, if you consider what it does. It does NOT do the deep cleaning a regular vacuum provides, but it makes a room "livable".
We love the thing, it works great, SUPER easy to use (a power switch and pressing one of three buttons), and it does a decent job picking stuff up. It is a tremendous time-saver since you can run it while tending to other chores.
It hasn't marked any of our walls, no harm to the floors as wjsteele mentioned, it's just damn nifty.