I also have floor heat and love it. The best part is that all your equipment stays at temp also, no more cold tools and parts. I keep mine set at a temp that is chilly until you get to working and then its perfect. I didn't insulate under my floor but the coils are sitting on a sand base and my slab is eight inches thick. I'm using a boiler and it comes on only about every two hours and then for a short time.
I have an overhead heater that I use when I bring in a cold project car to help warm the air back up in the shop quicker as my thermostat is near the outside door. I located it about 30 inches above the floor but don't know if that's the best plan.
I believe the key to a great shop is to insulate it well, I used 8 inch R-30 fiberglass and it is also surprisingly quiet. In the summer I don't even use an air conditioner, I keep the doors closed and use a dehumidifier.