I've seen them used but not used one. Left a fair bit of grout on the floor to be honest compared to just getting on your hands and knees IMO. The washing off always takes longer then and that's when you're getting wet so I'd prefer to spend a bit more time grouting than washing off. Just IMO.
I bet with lovely flat ceramic or even porc they'd work really well. On large formats though you're still making the grout go over the tile when it probably doesn't need to be on the tile much.
I can see them working wonders with quarry's with thick grout lines and obviously relatively small tiles to what you all fix these days. Bet they used them when doing factories in the time of pottery industry boom. They were all quarry floors. Easy to repair, and even remodel the factories.