Vapor phase corrosion inhibitors are not new (Common in military or industrial applications.  What is different here is that the technology has been repacked into a neat little plastic case with a self actuated timer and adhesive back and marketed to the general public.  I put mine in a box with several hand planes which have high carbon blades.These work by emitting a vapor which deposits on the metal surfaces forming a film only a few molecules thick.  Close inspection of the 3-in-One product shows some open cell foam peeking through the slot.  Foam is a common carrier for VPCI's  There are other common carriers - kraft paper, plastic film etc.  Do a search on "VCI anti rust paper" or "Armor Wrap Paper" or "VCI Emitter"Who really needs this?  Fishermen perhaps, where you are exposing expensive things to a salt water vapor.  Because it works by depositing a film on metal, I wouldn't use it on cameras or other optical instruments.  Musical instruments, jewelry, and silverware should be fine.  If you're just trying to keep your sockets from rusting, there are less expensive options.  Oh yeah, you need to keep it in a closed space or the vapor won't protect very well.It's a good idea but here are the following issues:1) This product is only "good" for 90 days.  Similar products are good for TWO YEARS - they lack the nice packaging that the 3-in-One product has, but are also less expensive - about a dollar or two apiece in quantity. (search for VCI foam emitter)2) Each time you use one you will be throwing a way the plastic case - it would have been nicer if they had designed it with a plastic holder, but the internal foam emitter was replaceable.3) This last one isn't an issue - 3-in-One missed the opportunity to tell you that these also protect electronics - these would be ideal in a computer you had on a boat or if you lived near the coast.  We had a problem at work with computers being exposed to sulfur (common in clay) some years ago and these would have come in handy had we thought of it (we ended up changing clay)Here's a cheap substitute - I put desiccant bags into my tool boxes.  These work differently (by absorbing moisture) and are not as effective, but you get them for free in many things you buy.  Just look for the little pouches that say "Do not eat" or "Dessicant - do not eat" and save them.Full disclosure: I received this item through the Amazon "Vine" programIf you've found this review helpful, please let me know.