Yes you WILL notice a difference, but keep in mind that your car and the filter are capable of demanding much more air than the stock intake can deliver (that little tube going into the fender sucks, no pun intended). Open that sucker up if you're going to go K&N. Several different ways to do this, some people just punch holes in the box, some add larger tubing, some go CAI with a cone. I only gave $40 for my filter on the net, so I kept it as cheap as possible. Here'* what mine looks like now:
Don't laugh at the gutted page.
Drifter and Tamara did it differently. Their intakes draw from down near the fog lights. The point is, that these things do work if you LET them. Don't waste your money unless you're willing to cut on your car. No the K&N won't void your warranty, but don't cut open your stock airbox if you still have the warranty. Go with a CAI, and put your stock box back in if you need to go in for service.
Yes, you'll see better fuel mileage, but only if you can keep your foot out of the extra power