I was coming back from yosemite and had my camera in the bag with the zipper unzipped. Just in case I wanted to shoot something scenic, I wanted quick access to the camera. Well, my wife was unloading the car and she pulled out the camera bag not knowing that it was unzipped. My 5D II came crashing down with my 16-35mm II attached to it. It landed on the filter and the ring was smashed in. It took away the majority of energy from the fall. It also bent in the filter thread ring on the lens. I took the lens to Canon Service Center and was charged a little more than $250 to get the filter ring replaced on the lens. Canon charges flat fee for the labor. In my case, I believe it was $150 which includes disassembly, reassembly, cleaning, and re-calibration. The filter ring was a little more than $100.
If I didn't use a filter, the lens would have seen more damage for sure. I use Hoya HD UV filter. If you are using a cheap filter, don't use it at all. If you are using a high quality filter, it won't make much difference expect when shooting into the sun.