My experience is that even with a cycling-specific rain jacket that was too big for me, if I got up any kind of speed, especially on a long downhill, it was a big flappy balloon. Loud and not real comfortable, but the worst part was when the shoulders got ballooned up it would interfere with my side/rear vision. That was when I realized cyclists wore skin-tight stuff for a reason.

If you're just biking around for errands, especially in warmer temps, a cape might be nice. I was curious and found a helpful thread here.