I got one and was not impressed. It was supposed to be "acrylic safe" but scratched hell out of the SO's tank. (lfs guy says that many tanks called acrylic are actually just plain ol' plastic, so this may not have been entirely the mag-float's fault.) And it only cleaned a little strip like 1/4 inch on each pass, so it was a substantial amount of scrubbing to clean even one wall of a 12g.
A fuzzy plastic scrubber pad works much better, imho, besides being cheaper. Yes you get your arm wet. /shrug