I had an Undercover solid tonneau on the 6.5' box on my last 2005 F150. It worked well for me.
Late last year, I traded for a 2014 F150. I wanted a tonneau, but Undercover didn't make a 8' tonneau, so I looked at several different hard, fold-able tonneau covers. Soft tonneau covers don't work for me since I always carry generators, spare gas, air compressor, etc when pulling the trailer. The only problem with the fold-able hard tonneau covers is I could never get one that didn't leak.
So I ended up with a Leer fiberglass cap painted to the color as my truck.
The Cap's biggest drawback is the weight. It will be very difficult for me to take it off, and it adds 200+ pounds to the truck all the time.