I've never heard a good story about them. Either people ruin them by letting two sticky parts touch and they become permanently bonded together, or they fall down when you're not home and two sticky parts touch and they become permanently bonded together.
There's a thread about them on a fantasy football board I browse and there's a ton of nightmare stories about them. Some guy's toddler came over and sat on one spread out on two chairs while the guy was preparing to hang it and in his words "turned the Fathead into a giant taco"...$100+ down the drain.
A helmet one would look cool on a wall though. I don't know if you can EASILY move it around like they claim on the ads though.