I threw one in the trunk of my car (just in case - I ordered a 4-pack and I had a spare one), and every single time I drive somewhere it chirps loudly when I'm exiting my driveway, making its presence immediately obvious without any delays, and despite my phone being with me in the car.
Nearly all my hardside cases have an airtag stuffed in one of the "Surface Mount" kits from ElevationLab. it looks like a pressure valve on the other side, and I might replace them with Security mounts if I'm really worried. Having those was a FANTASTIC way to track my cases as I left them with a (trusted) friend to be shipped along some other Very High Value gear. Being able to see what was going on (and know when it had reached its destination) was invaluable. On the way back, I could see my luggage as it slugged its way through the airport luggage handling system. It's not real-time but good enough for rough location.
A friend of mine was able to locate their stolen vehicle down to the block and then drone-find the vehicle from there, call the cops, and ended up busting an interstate chop shop in the process. The AirTag consistently got gasps of updates from passing vehicles and the neighbor's HomePod.
All this because she had hidden an airtag in the gas cap.
There are airlines that are encouraging people to put airtags/tiles/samsung trackers on their checked luggage because it helps them keep the airport handlers in check. A prime example of this is flying with guns (yes, you can fly with guns!) and how having an airtag made it EASIER to recover the firearm: https://www.youtube.com/watch?v=OHyb2amIkzo (This happened AGAIN, by the way: https://www.youtube.com/watch?v=GBngUc3rmY0 -- Yes, airlines are TERRIBLE about handling these things!)
That same company also makes the 10 year AirTag battery. I am surprised that no one has released a 3D model to print. It would be popular with DIYers.