I run my cap during the winter for dry storage. I also use it as a pseudo camper in the summer, throw an air mattress in and I'm dry when everyone else is soaked in their tents! My mileage does not change cap on or off but I also have one of the stepped up caps. I do have to clean the rear window quite a bit like mentioned above, but I tend to wash my windows often. It does limit my visibility a little, but my glass is slightly tinted. Only thing that slightly annoys me is that I have a one piece slider on my truck, and my cap has a two piece so there are 5 vertical bars in my rearview. If you are getting a cap, go out on craigslist or your local classifieds and look for a used one, you can save BIG bucks. Mine came with the truck when I bought it. I can move it around myself, but I need 2-3 people to take it on and off. Unfortunately my garage door is not tall enough to get my truck in with the cap on so i can not rig up a pulley setup in the rafters.