We have a Roomba 560. We borrowed an early model from some friends who went away for a year, and have now bought our own.
The model we have is compatible with the "wireless command centre", which enables us to drive it like a remote controlled car. This is pretty fun, and helps with getting at bad spots if we don't want to wait for the Roomba to do that area. (The floor coverage is pretty good, but time inefficient.)
It has virtual wall-lighthouses, so that we can either cordon off an area or have it do one area and then progress to the next area.
We can schedule our model to come out at set times and clean, though I haven't done this yet because I haven't figured out a good time for it to do so.
Ideally, you clear as much floor as possible before setting it going. In practise, it can cope with a lot of obstacles. It isn't so good at coping at cloth on the floor (eg clothing) or trailing cables.
You need to empty the (small) dust compartment after each or every couple of runs, and also clean the brushes. This isn't that onerous, and easier on the newer models. cjbk1 as a result, you would spot the tiny toys in the dust compartment. I've got an under 3 and a baby, so we haven't progressed to strewn small toys yet, but observation of other behaviour suggests to me that it would move things like lego men but not suck them up, whereas the small lego tiles or one-peg bricks might get sucked up (trying to find something people would know how big.)
I did find that on the occasion my husband set it off in the bedroom without checking the floor, it found a paper tissue, and shredded it, scattering bits all over the floor. It was like a churchyard after a wedding. However, it also very effectively then vacuumed it up when I set it off again (having emptied the dust compartment and cleaned the brushes, which I don't think was needed because of the tissue but because the husband had interpreted "the Roomba needs emptying BEFORE it gets used next" as "yeah, make it vacuum the bedroom if you like").
My toddler likes to watch (supervise) the Roomba but runs away crying if it gets too close. He's pressed the "clean" (go) button in the past and got a fright when it then undocked and headed for him. I believe pets can also be afraid of them.
Ideal for people who are naturally tidy and not too cluttered. Perhaps not so good for untidy lumps with small children, like myself. However, the former are probably houseproud and can be bothered to hoover themselves, whereas I much prefer to mumsnet while it hoovers for me, plus it means my husband has some enthusiasm for a household chore (his general tolerance for mess and laziness at avoiding chores is unfortunately higher than mine.)