Just throwing my two cents in...
I've used K&N filters in my last four cars (2007 Mustang GT/CS, 2004 Grand Prix GTP, 2001 Monte Carlo SS, 1998 Malibu) and had no problems with any of them. Since I trade often, the highest mileage was only 46K. However, my father had a K&N filter in his 1996 Astro Van and had over 140K when he traded it in and no problems.
I just added a K&N to my 2011 Edge Sport last week and I noticed a very slight improvement in acceleration, it just feels a little snappier. I'm very particular about my cars and usually notice any changes. Also, I just checked the mileage for the first time and got just over 27 on the highway at 70-75 on an 80 mile trip. I didn't check the mileage before.
My experience has been the same with the other cars, a little better throttle response and usually 3-5 mpg better than the window sticker on the highway. Never really checked city mpg. Overall, I would recommend them.
- Jason