I've been using K&N for years on different vehicles with variable results. My old 85' 500SEL and 87' 560 SEC felt great, mom's 400E was not as noticable. Then when working on my 94' E500, I talked to Renntech about some mods, including an enlarged airbox, in which they include some K&N filters. Renntech has a great reputation for reliable mods and extensive research and development precludes the majority of their work. If they trust it, so do I.
I went with their chip, stainless exhaust, filter box which gave me a definite boost in Hp and torque. Suspension went 17" rims and michelin pilot sports + a renntech swaybar. This package helped me dominate at a Audi club event at Road America (where shoving around brand new S4s came entirely too easy).
Bottom line, I like and trust K&N products, however, cleaning these filters is tuff and re-oiling them should be done carefully to ensure proper filtration.