You press a button and it runs around the room vacuuming as it goes. It comes with a docking station and runs on a rechargeable battery. Once it gets low on battery power, it will go back to the docking station and recharge itself. For the size of the rooms in my house, one pass lasts for an hour, then goes back to the docking station and recharges for a while.
It has a bumper and sensors in front so when it senses that it's approaching large objects, it will slow down, touch it, back up a little, turn and move in a different direction. Sometimes, if the object is thin (like a table leg), it won't "see" it until it's too late and ram into it without slowing down. But it doesn't go too fast, even at full speed, so it's not doing any damage to the furniture.
It can also sense an "extra dirty" area and will circle the area a few extra times to do extra spot cleaning. The noise level is about the same as a regular vacuum, maybe slightly quieter. It is loud enough to be as annoying as a vacuum cleaner though.
It does corners better than you'd imagine. It has a little spinning brush that sticks out of one side. This spinning brush pushes dirt under it so it can suck it up. But it still can't get things that are in the very corners of the room. My experience though, is that the corners of my rooms are less dirty than the rest of the room, so this isn't really a problem.
If you want to keep it within a certain room or area, you can either close the door or use a Virtual Wall. It's a little black thing that takes two C batteries. It sends out a signal outwards from it across a doorway or opening and the Roomba knows not to cross it.
If it gets itself into an error situation, it tells you what the error is and tells you what to do. IT SPEAKS CHINESE!!! Among 13 or 14 other languages. I thought that was AWESOME. =D
The first time it did my bedroom, it tried to turn out of a corner that it had gotten into. When it turned, it scraped the bumper along my floor molding leaving a black skid mark. I was upset at first, but the skid mark came off with a Mr. Clean Magic Eraser.
It can clean both carpet and flat surfaces. It will flip rug tassles around, but it won't actually get caught in them. It also knows not to fall down the stairs. It has a drop sensor, so it usually gets to the stairs, turn and move in a different direction. I once saw it go towards a flight of stairs at a very acute angle and one wheel went off the edge. It didn't have enough traction to move backwards and get itself unstuck, so it just said there was an error and told me to pick it up, move it somewhere else and restart. In Chinese. =)
It doesn't do as good a job as you can with a vacuum cleaner. Part of this is actual fact. When I run it again in the same room, I see that the collection bin does contain more dirt after the second pass. But part of this is just perception though, ie. it's the randomness of its path. If you stand there watching it, you'll get really frustrated with how bad a job it's doing. It looks like it's missing a lot of stuff on the floor.
But if you run it enough times in the same area, it will eventually get the entire floor clean.
I find it doesn't do really small rooms very well. I have a large walk-in closet that's about the size of a small room. It just runs around in the middle. I have boxes on the floor around the edge of the closet. I don't know if it's just that it sense my stuff differently than walls and is staying away from them. But it wouldn't go to the edges like it would with the other rooms. And it wouldn't come out of the doorway, even though it had a straight path out of the closet door. It got to the doorway, turned around and went back into the closet again. I thought that was bizarre. I finally had to just pick it up and take it out to do the rest of the room. =P
In the first week I got it, I started it just as I left for work in the morning. I came home and it was ready to go on the second pass again. I thought that was great. Since my model has a scheduling feature where I can set it to run at a certain time, I thought I could get it to do the same room three times in one day. Kick it off once manually in the morning, scheduled run mid-day, kick it off again manually in the evening when I get home from work.
But it set off the motion sensor of my house alarm. So I won't be doing that... =P