On a trip last year to Mongolia, we had to use a screw driver to open a can of pears. So, this year, I took about 20 of these for gifts for the drivers and "prizes" for the officers I train. Almost everyone I gave them to put them on their key chains immediately. It is good to practice with it before you need it, and watch out for the pointy tip!
I use these in the field to open tuna cans (when there isn't time to pack a lunch in the morning). Gets the job done and works great. I had mine attached to a necklace. The only problem is that it's ferrous so I had to take it off when using my brunton.
I have used these types of openers for some 20 years and have to say, they are simple, straight-forward, and work. You just pop on and manually move around the lid. Like all lids, in the end be careful of the jagged edges. This is the way to go!
I've had a smaller version of such opener and it was so compact and useful that I decided to buy this opener in addition, to have two. This opener is worse than the older one, because the flap cannot be fixed in either folded or unfolded position. Sharp edge within the can is thicker than I used to have with the older one, and the handle is too big for my hands (average female hands).
All in all, this opener works, but it is not comfortable at all. .