Have had my DIY garage door insulation for many years and, yes, it's really worth it. Double garage. Steel door. Adds only a few ounces of weight but turns a cold garage into a comfy work space.
I bought a couple sheets of 4x8, 2-sided, foil faced, foam insulation. Cut to fi t the steel door panels. Install with Liquid Nails adhesive at the corners and the middle. You must leave an air gap or condensation will form between the door and insulation material = rust potential. The west-facing doors does not transfer afternoon heat, either. Double points!
Did this over 20 years ago and it's still perfect. I can warm the garage with my KeroSun heater and work in a T-shirt.
Forget the "kits". You'll just pay more money for what I got on the
cheaps.
Shut up, Bengt
Monte