Updated 04/11/2012: I’ve reversed my opinion on using the Roomba on wood floors, especially in an old home. Lately I started to notice that the finish of the floors in all rooms no longer appears as thick and shiny as it did when we first moved into the house. This includes the rooms we recently refinished.
The final piece of evidence is the near lack of finish around some of the air vents, which are places where the Roomba frequently gets hung up. I’m convinced that repeated runs of the Roomba brush across the floors has eroded the finish much faster than would normally occur from foot traffic.
I’ve stopped using the Roomba on the wood floors but it still gets some use on the tiled areas.
About two years ago, just after we purchased our home, I bought an iRobot Roomba 415. It was on sale at Woot for about $120, which was a good price at the time. Even then it wasn’t the newest model but I was confident that it would work well.