I installed a K&N shortly after I bought my HCH-II back in August 06. I've had them in my last three cars and over all my expierence is it improves MPG as long as your not hammering the pedal.

I currently commute 91 miles a day ~50 miles highway ~40 miles back roads and I average ~70 miles per hour on the highway and I got ~48 MPG.

I say got cuz I also switched out the OEM hybrid suspension and installed the SI suspension, which IMO has also allowed me to achieve even higher MPG.