I'm over 300, and use our ladder when I need to check or service our roof. I generally stay toward the sides for the most support available. Also, I'm not at all shy to admit I'm generally on hands and knee when up there.
I should say though, that our ladder is well maintained. Being older, I went through it a couple of years ago, replacing all rusted screws, as well as the plastic end caps that do crack on occasion.
I would NOT encourage walk on the roof beyond what is necessary to service it/keep it clean and in good repair.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake