Wouldn't it be easier to pay a window cleaner to clean your windows?
I think these units are really only for windows that are nigh on impossible to reach, i.e. those above a conservatory, or high level windows. I don't think they would do a particularly good job at achieving streak free windows ...... but my opinion would be biased anyway, seeing as I'm a window cleaner :)
I expect they work on the same principle as the magnetic cleaners for aquariums and it is a right pain when the one you're not holding falls off - very easy to do if you move your hand too fast and the magnet bit can't keep pace with you.
Tried these. Found that the surface of the glass has to be somewhat slippery for the one you aren't holding to follow the one that you are holding. Sqeaky clean wet glass, and they didn't work - too much drag. Gave up, dumped them, and went back to ladder, wet cloth, and squeegee.