I admit that my experience is limited. We designed just one green roof as a demonstration project for a storm water authority, circa 2004. The utility especially likes the decrease in rate of run off. In fact we have two spouts side by side to demonstrate the difference between the vegetated roof and an equal area of typical roof. The difference is dramatic.
Lately I've been wondering, "Is it just me or has the din about green roofs died down?" The touted benefits list is fairly long.
Personally I am not impressed by some of the benefits.
My vote is that green roofs are not worth it; and my personal opinion is that they will go the way of Pet Rocks. But read up on Wikipedia and make up your own mind.
Image credit: installation plan by Human Nature