I like the idea and am seriously considering getting one.
Pros:
Light - sub 500 lbs
Can be left on permanently and used as a shell
Fully customizable interior space
Keep tailgate on truck
Retain use of bed - dry storage of items
Cons:
Hard egress - open door and climb over tailgate, or open tailgate from the inside
Dust - I'm not sure if the tailgate area can be sealed
Bugs - no easy way to have door and tailgate open and keep out bugs
Cost - It's pretty spendy for a shell with a bed
All in all I feel it is a better option for me than a roof top tent or a dedicated camper. Maybe Marty at All Terrain Campers will start making them?