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Nite Ize S-Biner MicroLockI, too, owned a Nite Ize S-Biner, and found it convenient to put my keys, keyfobs, thumb drives, etc, on -- except the spring was too loose, and things would pop off all the time.
Then I found the locking S-Biner. The larger #2 and #4 locking S-Biner had a piece of rubber on the spring that you could slide in order to lock. While this worked well to lock it in place, I found the operation to not be smooth at all, and difficult to apply leverage to such a small piece.
Ironically, I then found the S-Biner MicroLock, which conceptually does the same thing, in that it locks the springs, but does so with a single, rotating knob in the middle. This knob allows for considerably smoother operation and application of torque than the tiny rubber pieces on the #2 and #4 S-biners.
As such, this is now my favorite inter-link for my keys, keyfobs, etc. It works surprisingly well, the locking mechanism is quite secure, and easily the fastest when it comes to swapping things in and out.
The little secret extra value here is that while you can buy 2 MicoLocks for about $5, you can buy the Nite Ize Key Rack Locker, which comes with 6 of these MicroLocks for just a bit more.
I now have these all over the place, and enjoy the convenience of attaching and detaching things with ease, yet knowing that it will absolutely not randomly detach. The biggest impact this has had is on my backpack, which happens to be a hybrid city/camping backpack, which has lots of service loops. I can finally make use of the loops without worrying about the hassle of using keyrings, or the fear of losing things when using a non-locking S-Biner or velcro.