I used one for a few thousand miles. Didn't really notice a performance or mpg increase. A lot of our accelleration comes from the electric motor, which of course has nothing to do with the air filter. I did notice, though, that in cool weather the engine took a little longer to warm up. Also I've seen independent test results of multiple air filters, and though not terribly scientific, they did indicate that K&N filters let more dirt through. So I took mine out and put the stock one back in.
I have a K&N Filter (P/N:33-2180) in my 2001 CVT. In fact, I put them in all my cars. I have never seen any significant mileage gains but that is not the reason I use them. I like the fact that I never have to buy another filter again. I just check it during routine maintenance and, if it's dirty, clean it, oil it, and it's like new!
K& N filters and all filters that require oil are designed to work at their best when there is a layer of dust on them. The light layer of dusk actually serves as a critical filter layer that helps minimize contaminantes from penetrating the filter. K & N filters let alot of dust through specially when they are new/clean. My friend had done oil analysis that showed high silica count in his engine oil. The analysis lab recommended changing the air filter (and oil). He went back to an OEM paper air filter and the next oil analysis showed very low silica content so the air filter was doing it's job.
This is just one example, my mechanic has said he always sees dust in the intake right AFTER the air box when a K & N filter is used.
They do work as claimed for better air flow, but actual results depend on the car design. Fact is they let more dust and dirt through. This small dust will not cause any short term problems, but it will increase long term wear on the engine. How long do you plan on keeping your Insight?
I found a good alternative to K&N. I don't remember the exact name, but they were green.. very green filters. In contrast to the K&N which are made of cotton gauze the green ones are made of 100% cotton and specifically stated that they didn't let more dirt through like cotton gauze filters. I'm not sure whether or not they are to the market yet as I saw them at the SEMA show in october. I did look in their catalog though and they did have one for the Insight listed.
I did have a K&N in my Insight for a little while, but after reading how they allowed more dirt through I decided against one as we can get some nasty dust storms here occasionally. Speaking of which I probalby need to go get a new filter.