In my experience, a rain cape will keep you totally dry above the waist, and keeps you cooler than any waterproof jacket. It does slow you down a bit if there's a wind, but I usually find that it's not a big deal -- in the rain I'm taking it a bit easy anyway because of visibility and braking issues.

However, I commute in the city and a cape doesn't keep my legs dry. From the knees down I get hit by the spray from cars. Usually it's fine, but if I'm going somewhere after work and really need to be dry, I wear rain paints along with the cape.