I do! Well, not me, my DTDs (4yo) They had them as a present from their GPs when we went on holiday, along with Trunkis.
No, they're crap and pointless. The girls wore them as comedy accessories the day they unwrapped them, but they're rubbish in the car as they seem to keep their heads too upright (IYSWIM) and since their car seats have "wings" which them can quite naturally rest their heads on to sleep, they were pointless. On the plane, where we hoped the girls would use them to go to sleep, they were worse than useless as the back bit (where they're supposed to rest their heads) is floppy, so have to be sitting up against something, with the back bit in the right position, and then not move whilst sleeping, otherwise it flops down.
So, other than the comedy accessory bit, I wouldn't advise buying one.
Trunkis, on the other hand, are clearly made in heaven by fairies. I don't think we'd have made it to the plane without them.