I'm very happy with my O2 Rainwear pants: http://www.rainshield.com/p_cycling.html

They stuff into a small bag, so they shouldn't take up too much room in a pack. They are lightweight, and easy to just throw on in pinch. Certainly not for the 20mile+ club around here, but they work great for small utility rides under 10 miles.

But, they will make you look like a duck, and certainly aren't cool. Much less cool than full coverage fenders.