I used to use them on my old 3G and they were okay -- I ended up replacing them more often than other protectors I've tried. If you have a silicone case, sometimes it makes the corner of the protector pop up, which is irritating. It made them get dirt/dust under those corners so you had to replace it more often.
I've also tried the anti-glare ones and those fared much better -- never needed replacing, but when I took off the anti-glare I realized how much of the pretty screen I was missing, so I never went back.
They're *okay* for 9.99, you just have to watch which case you put over them.