I've been driving in ECO mode for nearly all of the 2 years I've had my Prius, assuming it would result in better fuel economy. After reading a post a while back (tried searching, can't find it) where somebody said their MPG increased when switching from ECO to normal mode, I decided to try it. It's been about 6 weeks, and in normal mode I'm getting 0.5 MPG higher than I did in ECO mode. Car lifetime MPG is ~45 FWIW. My driving pattern is the same, the only variable that's changed is the outside temp--but although it's warmed up a bit here, we've had unusually cold weather the last month so the temp change has been minimal, if anything.