I did a test on my Geo Metro recently and found that unrolling the windows @ 50mph only saw a 1 mpg (1.8% decrease in FE. In my opinion, it doesn't make enough difference to prevent me from keeping the windows open on hot days (since my car dosen't have A/C... and you'd bet that A/C would drag my FE down a lot further than just 1 mpg at that speed).
I actually want to take my dad's Intrepid out sometime and do the same test. Figure out the speed one has to attain for A/C usage to offset the aero drag caused by opened windows.
On my Saturn, I needed them for the rain. On the freeway above 35 MPH, I use them to crack the window on a hot day in LA. That way I get ventilation but I *hope* less aero-penalty. They definitely are worth it for me. I have a four door, but I only got two for the front windows.