As part of our recent home upgrades, we decided to invest in an insulated garage door, properly fitted and snugged up to the walls, with side and top sealing strips mounted externally to minimize air exchange between outside and inside. We considered insulating our existing double garage door, a metal economy unit, but it had been poorly installed (long before we bought our home) and would have required re-mounting to be more weatherproof. In addition, the old motor had ceased to function, requiring manual opening and closing. We decided that since we had all that work to do, it would be simpler and easier (although more expensive) to just buy a new, factory-insulated door and motor, and have them professionally installed.
We turned to Overhead Door, the original suppliers of such hardware. They're expensive compared to their competition, but their reputation is second to none, and they have an excellent warranty. Thanks to supply chain issues, it took five months to get the door here and install it, but that finally happened - and the results have exceeded all our expectations. From being a hothouse in summer and an icebox in winter, our garage now maintains a much more even temperature (aided by a single HVAC outlet we installed there when we upgraded the house system). It's a few degrees warmer than the house in summer, and will doubtless be a few degrees cooler in winter, but it's now a place where it's comfortable to work. It's also no longer a heat or cold sink affecting the temperature in the house, making the HVAC work harder to compensate. The improvement is so marked that I wish we'd done this years ago.
A more temperature-stable garage has the advantage that you can store more delicate items there (e.g. food reserves, etc.) with less risk of them spoiling or going "off". Another advantage: in an emergency, we can put mattresses on our garage floor and sleep a dozen people there in reasonable comfort, whether it's 100 degrees plus during north Texas summers or down in the teens during winter. Previously, that would have been unthinkable, as the uninsulated garage usually matched the outside temperature. In high summer, it was like a sauna (to the pleasure of our cats, but not Miss D. and I!), and in winter it was an icebox (sometimes literally - bottled water froze). Not any more.
I recommend very strongly that if you haven't insulated your garage door, and had it properly sealed to minimize the exchange of air, dust, bugs, etc. between inside and outside, that you do so as quickly as possible. It's made a huge difference to our home. Yes, the process can be expensive, but there are cheaper work-arounds. You'll probably need a garage door technician to re-mount or adjust your door if it's a loose fit to the wall, but that's a very important part of the improvement, so it's worth paying for. As for an insulated door, if you can't afford one, buy an inexpensive garage door insulation kit, available from most hardware stores or online. We fitted one to the plain metal garage door on our outside shed, and have found it's probably at least 80% as efficient as the purpose-built insulated double door on our garage. That sort of improvement is good value for money.
All in all, I'm very happy with the added "livability" the insulated garage door has brought to our home. I highly recommend it as a worthwhile home improvement project.