Some cyclists use ponchos effectively but there's a learning curve I haven't been able to surmount, and I've had better luck with jacket and pants. Good luck if you decide to go that way.

Over the years my focus has changed from staying dry to keeping warm while wet. I now carry only the cheapest, lightest rain gear. My current gear is a homemade silnylon anorak and pants, total weight seven ounces and it packs the size of a double fist. Key to me is wearing the right layers under it for the temperature and exertion level, while keeping enough dry insulation in the pack for descents and for camping. This often means putting on cold wet clothing in the morning and warming it up with exertion. And it definitely means eating, resting, and hydrating well in extended rain, and bathing when you can because the sweat really stinks. This system worked fine for me through a week of rain and wet snow in the North Cascades and Rockies last spring. Again, good luck finding something that works for you--it was a long process for me.