Good functional product. Compared to similarly priced (although bit smaller) Smallrig arm and clamp:
- The rotatable screw mechanism with ARRI pins in MS13 is better for most use cases than the screw and rubber ring on Smallrig. The pins retract and there is rubber for anti rotation for non-ARRI use as well.
- Magic arm ball locking is more reliable. Especially in positions where the magic arm ball rubbers engage, they make the locking very rigid and rotation resistant. On the other hand they make turning stickier and finding the correct position harder
- Magic arm balls have more freedom of movement, which is also why the position where rubbers don't engage can mostly be avoided
- Clamp feels bit more solid and has less play than Smallrig clamp, but doesn't open as wide and jaws have less rubber padding. When locking on a pipe or tripod leg, this clamp will lock with bare metal as the rubber pad doesn't reach far enough. Fixable with bit of glue and rubber, but a weird oversight.

Either one will do what they're supposed to. K&F MS13 one has significantly better magic arm, with better locking and movement range. Smallrig clamp doesn't require glue + rubber mod, opens wider and has one extra female 3/8” ARRI interface, so despite the slightly less robust construction I would rate it bit higher due the added versatility.