A K&N panel air filter was my very first modification back in April 2007 and my car only had about 800 miles on it at the time. I never really noticed any difference between that and the stock filter, but I wasn't romping on the car at the time, so I can't really say if there was a noticeable difference. All I know is that K&N filters are known for flowing more air, so if the engine breathes easier, it probably makes a little more power and gets a little better gas mileage. I cleaned the filter regularly (probably about once per year) until last year when I switched to a big maf intake. I actually ended up selling the filter to one of the members of this forum and it was still in like new condition.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)