Pretty detailed. I used to own one of the high-end Roombas that would show you a map of your house, and the wifi signal strength in all the nooks and crannies (really quite useful when you move to a new home).

It doesn't show what kind of chair you have or what you have on the walls, or anything like that. But it's not hard at all to determine the size of the rooms, the layout, the placement of key pieces of furniture, and similar things. From the way a home is arranged and the amount of furniture, I can easily think of ways that the map could be used to infer information about a home. (Larger couch and entertainment center may indicate someone more interested in consuming media. Individual chairs around a smaller coffee table — perhaps they like entertaining.)

The maps are far from accurate, since there are a number of problems with a Roomba building a full map: Closed doors; areas cordoned off by lighthouses; unless you put blue painter's tape over the sensors, Roomba thinks rugs with black in them are cliffs.

But the maps don't have to be prefect to be very revealing. Especially since the higher-level Roombas have cameras on them. I don't know how detailed the images they capture are, but even if they are only vague monochrome blurs, that's enough to determine patterns of activity in the house. (Family regularly comes home at 5pm? Start advertising dinner food on partner web sites and apps.)