I've heard the same thing as schulze. I guess one problem with aluminum foil is that it is hard to keep wrinkle free. I actually have aluminum foil on my light fixture, which I will be replacing. I could only find Mylar by the roll. That was $30 for 1 mil mylar in a 4' x 25' roll. They had thicker stuff that was $40 a roll, but regardless that was a lot more mylar than I would need, and more than I wanted to spend.
However, I went to a lhs (local hobby store), and found some self adhesive chrome. It's supposed to be for RC airplanes. It comes in 30" x 5" (approx) strips and cost me $2.79 ea. I bought a couple of them and will try them out in the next few days. I'm note real sure if I will see a difference.