I also have the Bak Flip MX4 and it is a great product and it keeps stuff secure and dry. I chose it specifically for four reasons:
1. It essentially is locked when you close the tailgate since it opens from a release under the cover, so I don't have to mess with carrying a key and it is always secured.
2. It can hold hundreds of pounds of weight, I think more than any roll-up ones (not positive about this statement).
3. It didn't have the storage box at the front of the bed like the roll-up styles so I could put an in bed toolbox up there. (I have since decided not to do that).
4. I can open it completely to utilize the full bed to carry large and tall cargo without removing it.
I have found a few things I don't like about it that if I did it again, I might choose a roll-up style:
1. It occurred to me today, that since it is locked when the electrically operated tailgate is closed, if I have a dead battery I cannot get to tools and jumper cables stored in the bed easily.
2. I would like to be able to see out of the back window when it is fully open, even if the cargo blocks my view.
3. I am worried that it will need to be fully open to pull a 5th wheel - again, blocking the rear window. I don't have the 5th wheel to verify one way or the other.
4. It is wide enough, and the panels tall enough, that if I want to put a bed rack on the truck, there is nothing offered that will allow it to clear the sides of any commercially available rack, and I would have to weld up a relatively unattractively boxy rack myself that will not follow the lines of the cab.
For how I am using the truck now, it fits my needs perfectly, but for some people with specific applications, the drawbacks I have found might be deal-breakers.
Good luck on your choice! There are lots of great options out there.
Ed