Mink Oil has been around a long time. I started using it in the late 70's as it was the only thing we could find for waterproofing leather boots. I've used it a lot. Its inexpensive and easy to find but there are much better options available. Mink Oil is not good for "waterproofing", its more water-resistant. Its fine for walking around in the rain for a short period. Over time though, water will get thru and soak the leather. I still have my leather hiking boots that I only treated with Mink Oil. If I wore them out in rainy conditions for several hours my feet got wet.
Mink Oil is decent for treating the leather. My old boots still look decent and the leather is in decent condition. The problem with it is that over time it will make leather too soft. Mink Oil can also get rancid and smell bad, though I've only had that happen in the container and not on the leather.
About 15 years ago I started using Nikwax for hiking boots and its much better at waterproofing. It seems to breath better also. Since the proliferation of information available on the internet though, I've learned about all kinds of products I didn't know about before and have been trying them out. I have Sno-seal and Obenaufs LP. I like these products better for strictly outdoor use boots. For boots that I want to treat without changing the color of the leather, I really like Bick 4. It seems good for treating the leather (without darkening) but only seems to be water-repellant. I also recently got some Filson Boot Oil to try for treating leather. I imagine it will be like the Mink Oil though and susceptible to over softening. However, for strictly waterproofing, Nikwax is still my favorite.
It's funny, I've used nikwax on my true hiking boots for years, but never really thought of using it on nicer boots. Does it change the finish/color much? Any downsides?