My experience with the Rainfly is that for $20 you can add an extra layer of water protection onto your prodigy bag. It heavily limits the accessibility of the side pockets and front zipper pocket and makes getting a bottle out a tad difficult. In constant and heavy rain, I don't trust this alone to keep my discs and towels dry, but with an umbrella, I don't worry about anything but the bottom of the bag getting any drops on it whatsoever. So, for the price, why not?
I’ve only used it a few times but it kept all of my discs and the bag completely dry in the pouring rain.
I haven’t had a chance to use it yet but it fits well.