thumpism 

Another vote for the poncho, especially when used with fenders. I've spent whole rainy days mostly comfortable and dry with that combination. In fact, I spent a couple of hours in a park in France installing fenders on my Batavus (in the rain) so that my poncho would keep me dry all over and not let me get soaked from below. I started out with an olive drab Boy Scout unit, fairly lightish, but when that one began falling apart I got a plastic Stearns brand one from W mart and that also worked. I've been meaning to trim the backside to reduce flappage; I would cut the square shaped back into a U and probably rig a belt glued to the inside of the back so it stays close in. Just hang the front over the brake levers and your handlebar bag.

I also have a Burley rain cape that is nice but is cut a little close to be completely comfortable.

I do not use the poncho's hood when riding, relying instead on a waterproof helmet cover that is less likely to obstruct peripheral vision.