I've been using these on all my guitars and basses for years now and I couldn't be happier. I actually started out using the "Flush Mount" version on 1 guitar and 1 bass when I first discovered the system, but went on to use the "Dual Design" on all successional instruments, because they make it possible to still use "regular" straps and of course because they're much easier to install and the mod is completely reversible.
What I like most about these locks is that there's no screwing around (pun intended) - apart from the screw that holds the pin of course. The counterpart that you attach to the strap is locked in position by a spring-lock washer that won't come off unless you use tools (like a small screwdriver) and a lot of force.
And should it not hold sufficiently anymore, because you have taken it out and put in in again (to adjust a "traditional" leather strap, to put them on a different strap, ...) too often, you can get replacement washers for a few cents at pretty much every hardware store. I've never had to replace one yet, though, and I've changed strap lengths etc. quite a lot over the past few years.
Also, when connected to the pins, the locks can freely rotate 360° so you're absolutely free to move. And you have to push the button (protected against accidental pushing by the collar) and simultaneously pull the lock out of the pin to separate the strap from the instrument, which makes it pretty much impossible to open these by accident.
I haven't had a single incident in about 6 years (using them on 11 instruments).
