They are fairly simple for people who know what they are doing, done properly they will look great, the only time people have become unstuck with them is when they or their fitter have no experience of PU and no concept of how they are supposed to be fitted.
They are flexible and mouldable with heat, 9 out of 10 times they will fit with very little persuasion but they can distort slightly in the box or if left unfitted for a long time. When this happens all that is required is some form of heat to return it back to shape, hair dryer/sunlight/heat gun. This will make it more flexible, then for best results bond/screw it into place and then prime and paint it