3-IN-ONE No-Rust Shield Rust & Corrosion Inhibitor works by emiting Vapor Phase Corrosion Inhibitors (VpCI) that form a layer just a few molecules thick over metal surfaces. This layer acts as an invisible shield to protect metals from rust. It looks like a fancy device with a "gauge" for indicating when you should replace the "cartridge." In actuality, it's just a piece of specially treated foam encased in a plastic shell. The gauge is simply a chemically-driven indicator that changes color from one end to the other over time.Pros----+ No messy greasing and degreasing, for example, of fire arms.+ Vapor penetrates cracks and crevices to protect out-of-reach or hard-to-reach surfaces.+ Accurate gauge that tells you when to replace the cartridge. I started using the 3-IN-ONE sometime in the beginning of September (just over 30 days ago). The gauge is now correctly showing it has just under 60 days left of use before it needs to be replaced.+ Easy removal of 3M Command' adhesive strip.+ No-hassle protection (other than the need to replace the cartridge very 90 days).Cons----- Needs to be replaced every 90 days.- Cost of replacing cartridges over a lifetime of protection can really add up.- Works only in confined or enclosed spaces.---Whether the pros outweigh the cons or vice versa will depend largely on what you use the 3-in-one Oil Rust Inhibitor for. Personally, I wouldn't recommend it for protecting the stuff you plan on putting away in storage for long periods since you will need to remind yourself to replace the cartridge every 90 days. As it is, I'm sure we all have enough to worry about in our busy lives, and we really don't need to be tacking on yet another task on our to-do list. So, for long-term storage, a light coat of oil might be the way to go. For everything else that you need to use fairly frequently, the indicator will serve as a convenient reminder to replace the cartridge when necessary. In this case, the 3-in-one Oil No Rust Shield Rust & Corrosion Inhibitor definitely merits some consideration. Ultimately, you have to decide whether the 3-in-one Oil Rust Inhibitor is practical and economically feasible for you.