I just started looking into this myself.

First, I would not use the spray on type product, as it seems to me it
would be a hell of a mess to apply. Also, I'd be concerned about it's
effectiveness vs the solid foil type product. I'd go with the radiant
foil type products that have a sheet of aluminum combined with a sheet
of polyethylene plastic in a roll 4ft wide. Seems like that would be
easy to apply.

Second, it does raise the roof temp. I saw some DOE numbers that
indicated it raised the temp from a few to maybe 8 deg. They said the
effect on roof life is unknown, but it doesn't sound too bad to me.
Like many things, there is a tradeoff.

Third, I think a lot may depend on how the attic is ventilated. With
the foil, you leave about 4 inchs open down at the soffits and 6 inchs
open at the peak. That should work really well with a ridge vent.
However, if you have other type of venting, like gable vents, then it's
obviously not going to work as well. I would think all the hot air
being channeled by the foil up to the peak could increase the temps up
there quite a bit.

I'm seriously thinking of adding this to my attic. From everything
I've read so far, it looks like it can make a significant difference in
the amount of heat that reaches the ceiling. And it's relatively
cheap. You can get 125' x 4' of the best stuff for $120. The cheaper
stuff is about half that. And I think the real diff btwn the two is
the better one is a heavier gauge material, so it won't tear when you
try to install/staple it. In terms ability to reflect heat, I think
the two are the same.