I've just tried over a period of two days a cheap poncho during conditions of light rain and moderate winds. I'm quite pleased so far. It did slow me down slightly, but really it wasn't all that noticeable . It kept my upper torso quite dry, with little to no sweat build up. My legs of course were wet, but that was expected. In a side wind occasionally the upwind side would blow up and expose my waist to the elements. I felt like I presented a profile that was roughly a foot wider as the poncho flapped around my back. That was not a problem for me, but it could be as when I passed through a narrow passage I was concerned it would snag, but never did. I'll give it another try during our next rain.

I'm also convinced the poncho is the way to go! I'm glad the OP is trying this time-tested idea! I commute in wet weather with a bike-specific poncho, and I think it's the best solution for me when it comes to breathability vs waterproofness. The only thing that separates a regular poncho from a bike poncho is that there is a waist belt to keep the back from flapping upwards, and hand "stirrups" to keep the front from flapping upward. Add fenders with low mud flaps, and occasionally gaiters, and I can ride in the rain while staying dry from both rain from above and sweat from within. I got my poncho from Center for Appropriate Transport Ultrex Rain Cape » Center for Appropriate Transport

im in Kansas so maybe my experience with ponchos is different. but about a week ago I wore one in some nasty wind/rain where the rain was going horizontal and I was regretting not having some goggles or something, but the poncho kept my torso relatively dry/warm but my arms were exposed and by the end of my 7 mile commute that had 50+ mph head winds my arms were cold. but on the non windy days in the rain the poncho has been good and if ya get flying at a good clip you might think your superman with the poncho flapping in the wind behind ya. but ive started lookin for a better rain jacket I still have the poncho for the emergency rain gear but id like something alittle better for future rides.