I've had the MX4 for close to a year and no dings or problems. I am conscious what I place on top of the cover, but I put hand and garden tools, trash bags, lumber up there all the time without problems. I am careful to set it, not drag it. Never been though a hail storm however.
There are a few drops of water in the bed after a heavy rain, but less than I would expect from the wide tailgate gap at the corners. Never got around to putting a foam seal there as others have done.
It works with my RAM boxes without any interference. Since it sets on rails between the RAM boxes it is virtually flush with the tops of the boxes, so it doesn't look like an afterthought.
The only minor thing I have noticed is that the distance between the MX4 RAM box rails from side to side is less than 48 inches. This means to load full plywood sheets, they have to be loaded below those rails, meaning the tailgate has to be down. Not a big deal, it just means I put a strap across the end of the sheet to make sure it doesn't accidentally slip out.
This is my first truck bed cover and I wasn't sure if I'd like it, but I wouldn't be without it now. I'd buy an MX4 again.