yes.. i think that mylar is about 95% reflective, meaning that of all of the light that gets to it, 95% of it will be reflected back to your plants.. also, flat white paint is about 85% reflective, while semi gloss is around 80% or so... i just use flat white paint, and it works much better than one would think...
some people say why not mirrors ... mirrors absorb light, and don't reflect it very well as a result.. and alot of newb's try tin foil.. the problem with foil is that is has a tendency to wrinkle, and when it does, the wrinkles create hotspots that can burn your plants..
i have heard good things about a product called panda film.. i believe its white, but i don't have any numbers on percent of reflection on it for yah..