I have one of these:

http://www.bicycleclothing.com/Rain-Capes.html

I climb hills on my commute, and I sweated with this almost as much as with my jacket (same brand, also made in Portland), on my arms and my back. It works, but I used rain pants with it as I go pretty fast down the other side of the hills I climb. Lots of road spray, even with my big fenders. I do a multi-modal commute and a jacket is more conveniant for me when putting the bike on a bus rack or just carrying stuff around.