Re:
Don't get me wrong they work when the tyres are off of course. I do, but they get caught on spokes and useless for cassettes ;-)
But for grey rim pad gunk etc, they are great as they don't damage tyres or act as solvent with stuck on rim tapes or certain nipples.
That and you can leave a pack in the care for a quick cleanup after a ride, yourself and the bike and they're cheap.
In find them beter than a damp cloth, but you'll not get the full wheel clean as in the above cleanup. Which looks great.