Ive had them on my back due to a growth spurt (5'10" x 145lbs). They're around 30-40% visible compared to when they first appeared. Pain in the ass when you have a friend that always has to remind you of them when your shirts off.
Once you get them they can be hard as hell to get off (stretch marks in general). The above is more for prevention (topicals). Once you get them you're probably going to have to up the product to get them off IMO. Ive been looking into dermarolling for my back, but it's not something you want to do for the penis