So I bought some of the Wet-n-Forget (Exterior version) and decided to give it a go. Remember, the goal is KILL the mold and mildew colonies, as well as eliminate the black and green stains. The product claims to do that. It also is a slow acting product--read, don't expect immediate results. It also is persistent, it hangs around for a while to keep the mold/mildew from coming back. If you have that ugly black streaking on the roof of your house, you can use it there. In theory, it will kill all the bad stuff and keep it away for a year or two before you have to reapply.
Pics to follow.