It turns out not everybody who buys the I-Pass actually gets around to sticking it on the windshield. Illinois Toll officials say they have seen people just wave it around as they approach the toll arches. To educate the public, the Illinois State Toll Highway Authority has put together an instructional video -- which is hilarious in the way only government-made instructional videos can be....
For one thing, it has very exciting music. It could be the "before the big chase" music in a Steve McQueen movie. It shows the WRONG ways to handle the I-Pass, each illustrated with a big red circle with a slash through it. It also is very useful -- I finally broke down, bought my I-Pass, and put it up on the windshield after I saw the video.
To watch the video, go to the Illinois tollway's main page and look to the bottom right.

Mine was on the windshield. When some rocks fell on my car from a truck in front of me on 294, I had to replace the windshield. Finding adhesive strong enough to hold the IPASS is a challenge. Unfortunately, the IPASS website doesn't sell replacement adhesive. Any suggestions?
What these fools fail to understand is that for those of us who live in highrises and park in valet parking lots, it would be idiotic to sitck an I-Pass to our windshields. It's too easy to forget them even though they come off with from their Velcro attachment strips. To a hiker and others roaming the garages by your car often left with unlocked doors, it's like a beacon saying, "STEAL ME, STEAL ME". This is particularly true now that the ITA seems to feel that $50 is the mimimum balance one must keep on a transponder, even if one only uses that much to go see the folks in a year. I bought a "Sticky Mat" for my dash and that is where it stays while in use. I've never had a problem. Otherwise, its locked in the trunk.
You should be able to get the replacement adhesive stickies at any of the manual toll booths or at the main office.