Well, I've had my discs on for a couple of weeks. Not noticeable gains in fuel economy ... but I don't have any apples-to-apples comparisons to make. I did just take a 320 mile trip (about 38.8mpg so far) on Saturday but the air temp was about 60F compared to my July 4th trip when the ambient temp was at least 80F (42.7mpg). I drove a bit faster ... and there was some city mileage mixed into that as well. I also won't be able to show any gains as the temps go down throughout this month and next. If my mileage merely stays consistent for the next 2+ months, I will feel I've accomplished some gains.
One serendipitous effect of the covers is the tremendous cosmetic appeal the car now has. I get people staring at me, other cars slow down to take a better look, people ask why I put them on, etc ... My ho-hum Civic has become a conversation piece.
One weird side effect is that at low speeds and with the windows open, I can hear a tick, tick, tick sound as the wheels go around. Oh, and the day after my 320 mile trip, I went around and found they had loosened just a hair. Minor points ... but worth mentioning.