Some complain about the strength of the bed. Granted it's not as strong as metal, but like Dcaplan, I've tossed tons of firewood/rocks in there without any issues.
There was just a thread about a guy's cleat breaking, causing his dirtbike to fall over and the foot peg cracked/punctured the bed, but even if he had a metal bed, the amount of weight/force focused on one spot probably whould have torn right through the metal anyway.
The composite bed is lighter than metal and will not rust out, which is a typical problem with metal beds. And unless your throwing cinder blocks from 5' away or dropping 2,000 lbs of rocks from a front end loader (typical truck commercial) then I don't foresee any problems. That's just me though.