Question about reflection

Ive heard many say "foil" is no good causes hot spots ect. I have a cfl grow and i need to make some sort of reflector for my lights ( 7 26w cfl's). From review Ive seen the "coke can" reflector mentioned many times. Seems like a great idea to me but whats the difference from this and regular "foil", there both aluminum, why does foil cause hot spots but "soda can" reflectors dont?

foil crease and that causes hot spots. If you have the money go to Home Depot and get the work light reflectors, I think they are like 6.00 or so.

thnx. that makes sence,but i think ill go with the can reflector's. i need 7, much cheaper for me this way.