I buy all of my hiking boots 1/2 size up and put the hybrid Dr. Scholl's gel insoles (the ones with the honeycomb center in the heel) right on top of the insole that came with the boot.
The thin boot insole will mold to my footprint through the layer on top, helping lock the gel into a stable cushioning footbed. Some people claim that the gel breaks down too quickly, but that hasn't been my experience using this method.
I hiked ~ 26 miles at the south rim of the Grand Canyon and probably another 14-ish miles between the North Rim and Bryce Canyon this summer on vacation and didn't have a single blister or hot spot.
And yes, they definitely make a difference. I used to work a job where I was on my feet on concrete 10+ hours per day, my feet, calves, and knees would all ache by the end of the shift.
Doubling up helped eliminate a lot of the fatigue.