That was my experience too. By the time I had written, and re-written the sheet, I didn't need it any more.
Also, if I couldn't figure out how to distill the entire course onto a single sheet of paper, I was going to get killed on the exam. That's one reason why I majored in math and physics.
Wish we were allowed cheat sheets for our physics exams, would have made things much easier!
(regardless of how many times I write down some of the equations they just won't stick)
Never seen the cheat-sheet thing done over here