I watched this video a few months ago and in their experiments they didn't see a noticable difference.

You also have to consider where our air filters are, since they sit on top of the engine they collect a lot of heat. So by just moving the filters you can make a difference in the intake temps.

I had a Scion TC TRD edition before my xfire and my mileage went up after I installed a CAI. with the cruise set at 90mph and no AC in the winter at 60F I got 26mpg over 700 miles.

In the summer in Kansas we had a lot of days over 100F, I once again set the cruise at 90mph and had to run my AC full blast and I got 27.5mpg and drove a total of 400 miles.

These were my findings so I'll still believe that all things considered a CAI does more good than harm.