I've been on the prowl for a decent light weight, easy to don, rain jacket for the days when I get dumped on and I think I found it. The Paradox Outdoors Vital Rain Jacket. I'm pretty impressed with it.
http://www.paradoxoutdoor.com/mum0019-2769.html The website lists it as a suit but I was able to score just the jacket at Costco last week for around $30US. I've found that it compresses quite small for packing, can be worn over my mesh jacket when it's raining, or under the mesh jacket on cool days as an insulating layer. Most importantly, it breathes well. Like most rain gear, it can start to feel clammy against bare skin, but once you're moving, the clamminess goes away.
I have several different rain jackets, ranging from a high zoot LL Bean Goretex job to a decades old Black Sierra pull over, with a few questionable choices in between. Most were chosen for bicycling or backpacking and don't transition well to motorcycling. The Bean jacket would have been a first choice, but it does not pack well. The Black Sierra, well let's just say it shouldn't be worn in public anymore.