When I built my garage, I was pretty hyped about in-floor heating.
Yes a warm floor would be a godsend if you spend a lot of time lying or kneeling on it.
The downside is that recovery times from opening a door would be considerable, and might even require a forced-air unit to re-heat the air.
After studying all of that, and weighing the costs and risks involved ... I was persuaded by a few friends to simply install a radiant-tube heater, along with a couple of industrial-type ceiling fans.
The radiant heat warms ME more than the air, but it also warms things like windshields and car bodies ... which in turn help to warm the air. My garage is well-built with 6" walls, vapor barrier, and R20 insulation.
The ceiling fans help to circulate the air under the vehicles and to warm the floor to a relitively comfortable temp. They also keep the windows clear.
In summer, the air movement over the cool engineered concrete slab helps to keep the garage at a fairly consistent and comfortable 20°C (70°F) temperature even during the hottest of days we get here.
It was relatively cheap to purchase and easy to install, and very economical to run as well. I heat my garage continuously as I have pets that live out there, but friends run theirs only when they plan to use their garage. It only takes about 1/2 hour before it gets comfortable enough to be out there.