haven't read other responses, but i love Power Grips. they haven't worn out yet and i can use them with any shoe i want, though i have them adjusted for a particular shoe.
at first i thought $30 bucks was too much to spend on them, but i don't think so now. sometimes i think the simpler ideas are the more brilliant ones.
(I only wish i'd invented them!)
EDIT: oops, i reread the OP and i see it's a question of using only the strap. sorry...