Well, I went on a short tour, only 8 days, to Bavaria, where they've had the wettest summer in years, and so, as you may or may not know, I got a Carradice Poncho - having previously had a gortex type jacket, which was slightly better than useless.
We ended up only getting one day of rain, and not constant rain at that.
But I have to report, the poncho worked wonderfully. I stayed dry, a constant stream of air passing underneath. I could feel the circulation, which meant I stayed cool and dry. There was no real wind, so can't say how it affected drag/billowing, but on one section we were on the road with lots of trucks, and I didn't have a problem with the poncho whipping up.
I even found that when it stopped raining it was still an effective top cover.
I found that if I got hot, I could pull the poncho right over the handlebars, so it increased circulation, and when I was a little cooler, I could drop it off the handlebars, thus lessening the air circulation and warming me up.
I'm really pleased with how it behaved and will definitely use it again.
One other thing - I stopped using my waterproof gaitors - they made my feet hot, and I found that after getting some fenders to fit my bike, my feet were covered from above and close by.