I was curious myself, as I always thought this stuff was snake oil. I found a Pretty good Q/A here: http://www.jlconline.com search for "radiant barrier" and click on the first story, it is free.
Radiant barriers are only worth considering for places where you need A/C, not for heating.
Basically, to paraphrase the article, by a florida building scientist; a radiant barrier has the potential to block up to about 40% of the heat coming in through the attic. If you have R-30 insulation up there, then there isn't much heat coming through, so blocking 40% of a very small amount won't be worth the bother or expense. If you have R-19 attic insulation then there is enough heat still coming through that blocking 40% of it could result in a cooling energy savings of 10-12% overall.
The above is from the article, not from my personal knowledge or experience. It sounds like if you don't have much insulation that it might be able to pay for itself over time. I guess you need it to be towards the outside of the house from the insulation. To my mind, it would seem that it might be easier and cheaper in many cases to just increase the amount of insulation to R-30 and forget the tin foil.