all good comments so far.....
yes magnetic stripes should really only be used if you can't decide, if you decide you like them then get the real deal
yes they can come off a speed, if not fitted correctly, I had this happen to me, was amusing seeing the stripe hit the car behind that had been tailgating me
yes they can mark the paint in two ways, grit, you must ensure there is nothing between the stripe and the paint, as this will act as an abrashive and scuff the paint work. The other way is that when they have been on for some time moisture will build up, and the laquer will look misty so you have to remove them every few weeks to allow the paint to get air and this will prevent the marks been permanent.
I still have mine, they are a now a fridge magnet