I've used various types of screen protectors in the past, and they have a varying tendency to leave bubbles which are impossible to shift. I'm sure this is because, after you clean the glass, the static attracts dust particles, which become bubbles when you put the protector on top. I have also had problems in the past lining the protectors up accurately on the screen, which causes them to become unstuck around the edges.
However, these protectors have a couple of advantages. Firstly they are soft which seems to help, and secondly they are inexpensive and in a pack of 10 so that you can have another go if you make a pig's ear of the job :-)
I'm happy with the result.