First let me say that I agree with people who say that the robot does a competent job of cleaning. You should SEE the dust bin after vacuuming! I am also in awe etc etc, but I'm going to try to focus on things that others have not mentioned.
My 550 (560 actually, I think...it came with a few more odds and ends in the box) is actually my second Roomba. The first was the Discovery SE, one of the 400 series. The 400 was pretty good with two exceptions: 1) It would not work on rug fringe. It gets hopelessly tangled in less time that it takes to say "Wait!" The second problem with the older 400 series is that the battery dies fairly quickly if you do not use it VERY frequently. I use it once/week and by second battery is showing its age.
In any case, the 500 series is advertised to be able to deal with rug fringe, and indeed it does so reasonably well. It is also supposed to be able to do 4 rooms using these "lighthouses" that have been previously mentioned. To summarize, these are little gadgets that put out an IR (UV?) beam that the robot can see; it generally will not cross the beam, so you can use the beam to mark off rooms. After a certain amount of time, the robot sends out an RF signal to the lighthouse, and the LH lets it through into the next room. Repeat for 4 rooms if desired. Then the robot knows how to find its way back to where it started to plug itself back in to recharge. Cool.
Here are the problems with the lighthouse scheme: First, the beam gets wider very quickly. So there is a fan-shaped area in front of each lighthouse that never gets cleaned. In other words, when moving between the two rooms, the beam gets shut off but when cleaning either side, the robot will not enter the fan shaped area that the beam covers.
The second lighthouse problem is that the robot seems to use time rather than room size or complexity to decide when to move from one room to the other. In other words, for larger or more complex rooms, it simply does not cover the room before moving on to the next.
A non-lighthouse problem that you should know about with the 500 series is that I think they must be lower-slung than the 400 series. My 500 robot is not NEARLY as good is climbing up onto a rung as the 400. The 400 got stuck in fringe, but without fringe, it is nearly flawless. The 500 tends to push the rug around, bunch it up, and even get caught if it bunches up the rug in a corner that it has not way to get out of without getting back over the rug.
You MUST be sure there are no cords on the floor. For example, a floor lamp, a computer power cord, etc. My robot has gotten caught when I forget, and nearly pulled down a few items with dangling cords.
And finally, I have one room with hardwood floor and a rug about 6 or 8 inches smaller than one dimension of the room. So there is a 6+ inch "valley" between the rug and the wall. The 500 series does not clean the 6+ inch valley well at all. It goes along it, but does not pick up dirt well. I think it is probably that the brush is on the rug on one side and on the floor on the other, but is touching nothing in the middle. I end up taking a dry mop and going along the valley and pulling dirt out into an open area where the robot can get it.
Given all that, though, the robot saves a LOT of work. If there is something special happening, I would probably bring out the hand vac and get all the little corners and fans and valleys that the Roomba does not reach, but for every-day/week vacuuming, it is way easier than doing it by hand despite its problems.
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Update 3/24/2009: I've had two problems, both fixed/replaced under warranty. First, it refused to dock. It would nose up to the dock and keep pushing and pushing; then finally back up and try again. Repeat forever. Used email support (I think). After doing a bunch of tests that they requested, they sent me a new dock. All ok now.
Second, the robot started spinning in circles. Even if I used the remote to try to make it go straight, it would turn. Sounds like a drive motor on one wheel died or something. In any case, I think I phoned this time. After a bunch of tests, they had me return the robot (minus dust bin and top insert) and they replaced it.
Now it works well, and guess what? The new robot can get up on rugs much better than the old one. I don't know if there was a hardware or software change, or if my old one was defective in some subtle way that I did not know about. I guess I might give this new unit an extra star if I could.
Interestingly, my 400-series also had to be replaced once, but it has worked fine ever since. I hope that will be the case with this one as well.
