That difference in temperature with radiant barrier is not an illusion. As our house was being built we walked into it on a 100 degree day and the temperature dropped twenty or thirty degrees.
My electric bill is very low due to the radiant barrier.
I just finished a major renovation on our house. Rebuilt the entire roof and added the radiant barrier decking and a wrap around porch. Took the outside down tho the studs, all new sheathing and Tyvec wrap and brick. We replaced all the old builder grade windows with double pane energy efficient windows.
My electricity bill dropped from $390 a month to $145 a month. I knew it was going to drop, but I was rather amazed at the results.