This my first experience with K&N products, and am glad to say I'm happy with the improved performance in my Civic (06 VTI-L automatic). Prior to installing the filter my Civic had the 100,000km service, and has been always serviced every 10,000km.
Firstly, while my Civic still has some go, it seemed sluggish from a stop start and seemed to react a bit slower when I wanted it to jump. That has much improved since adding the new filter. And it has improved the performance with general during drive responsiveness as well, now it's pretty fun to drive when I'm running a bit late.
Economy wise, my daily work commute is just over 30km one way, and involves some flat, but mostly up and down stretch of road at 80km/hour for around 15min, then a mostly flat 110km/hour stretch of highway for around 10min, combined with some parts at 60km/hour (with just myself in the car). On top of the work commute it also gets driven in my local area at 60km/hour on the weekend, and there are a lot of round-a-bouts for intersections in my area (with just myself, or with the wife and 2 kids, or myself with golf clubs in the car). Anyway, after initially filling with 95RON from Caltex, 9 days of commuting and a weekend of around town driving, adding up to almost 600km, I filled back up with slightly over 36L's, which equated to 6.17L per 100km. I'd say this would have been an improvement of .5L per 100km on the work commute, which does add up when I normally need to fill every fortnight! For those in Australia (SE Qld), my work commute is from Noosaville to Nambour, via Eumundi, and the local area is Noosa.
I have no doubt that if in typical highway conditions (100-110km/hour) that I could get close to 1,000km per 50L tank, depending on who or what else is in the car with me.
The K&N filter has improved the performance and economy of my civic, now I'm going to start looking for one for my wife's car.