Never been into 'em. I've got one, family heirloom, but I never use it. That means, in my line of work where a certain amount of vegetation management is involved. As others have said, if any trail maintenance, veg. management, pruning is needed, there are the right tools for that. (I've got them all.) But it never occurred to me to bring anything but a pocket saw on a recreational trip, and that, only for the rare occasion when a fire is appropriate.
As a self-defense device? I don't know.