I have never seen/heard anyone with theirs painted. I bet it would look pretty good, but it does add to the necessary black trim pieces (tires, wheel wells, windows, window trim, etc.)
I would agree with that, but I rarely haul anything around, unless its a few smaller items now and then. So, like Fast Eddie said, take that into consideration.
The bed is not constructed like a normal trucks. The plastic you are talking about trimming is actually part of the bed. It is not a bedliner and there is no steel underneath. Someone here posted photos of the bed disassembled about a hour ago. I wish I knew who had them so you could see what you are up against.