I have driving with it on for months now. In town, with traffic I actually get better gas mileage. But, I have noticed that on the freeway (after merging and settling in with traffic) I get better gas mileage with it off. I think ETC holds the RPMs up higher, even in 6th gear. Not a lot though, probably .5 - 1.0 mpg difference depending on distance. Maybe the longer the trip it would increase though, I only commute about 20 miles, and the commute back is bumper to bumper.
Definitely snappier shifting in town with it on though without requiring you to really hit the accelerator hard. I like it.
regards