I insulated my barn shortly after it was finished in 1985. I used the 3" batting (an R-19 I think?) in the walls and the 8" attic blanket (R-30) between the upper & lower sections. It feels like the barn has A/C on hot days if you keep the doors and windows closed to keep the heat out.
If I leave the stairs open you can feel the heat radiating down from upstairs so I keep the upstairs closed off from the main floor.
I want to do the garage at my girlfriend's house when I can. I know it'll make it a lot nicer out there for me.