They are a piece of pie to fit, the glue runs along the top and is more like the glue that you find on envolopes - its not very good either but mine have been fine so far!
They shouldn't do any damage
so long as you fit them correctly especially the metal pieces that come to help lodge the deflectors in place. If i remember rightly the spike bit needs to go facing outwards otherwise scratchy scratchy! (you simply put them onto the window and making the window go up and then hold it there and then bring it down and the metal piece should now be in its rightful place. The diagrams will help show you but they go in either corner of the deflector.
One ofther thing - Leave it for atleast 24 hours (fully up) rather than 12 it recommends, personally I find you get better and longer lasting results and helps everything get a bit more settled (all that rubber etc in the seal)