I have the K&N - filtration is supposed to be slightly worse that standard but the back of my airbox is as clean as it was when I had the OEM one in there. No gritty deposits, no mass of filter oil on there etc.
It makes the standard recirc valve slightly noisier apparently and increaes induction noise a little bit - can't say I noticed either much but then I have a UK with the slightly beefed up insulation, noisy exhaust, stereo etc and rarely open the windows.
I bought mine because I can clean it when I want and pop it back in again, it represents a saving over the long run, if you want a performance increase or more noise you need to follow another path IMO.