I actually paid someone else to do it because I've had really bad experiences with stick-on stuff in the past, and I wanted to make sure this was done right. As soon as I handed the covers to the guy he (correctly) predicted that bubbling on the top (horizontal) surface might be an issue because the plastic is so incredibly thick (like 1/8" or more maybe). Since our headlights make a 90 degree turn from the vertical to horizontal surfaces, the thickness of the plastic naturally pulls the smaller surface area section (the area around the horns) away from the headlight surface. The only thing we could think to do would be to either trim down the area around the horn (upward slope near the horn was a problem too) or use something even more sticky to hold the top area down.
It's because of this issue that I'm looking into clear coating my headlights. If someone can figure out how to beat the bubbling issue I'd change my mind though.