I had a pair of danners that squeaked where the leather rubbed between the tongue and uppers. I lined the uppers with adhesive fleece where they made contact and solved that problem. (back of the eyelets)
Yup, I put it on almost all of my boots. Love it. Extends the life of my boots by quite a bit. But I almost exclusively buy boots that can be re-crafted and re-soled. I'm not sure if it would be worth it on cheaper boots that cannot be resoled. Also, a little secret: The boot guard is pretty much the same thing as DIY truck bedliner!
It has been my experience that squeaky leather is due to the leather being dry and needs to be treated with some sort of dressing such as Nikwax, Grangers G-Wax paste or snow seal beeswax or some other depending on the type and brand of boot you have. Steer clear of products like saddle soap, mink oil, or other that say will soften leather as you will loose support around ankle.