I bought two sets of these to retrofit to a cheap mandolin that I bought, which had horrible quality plate tuners. I needed something small and compact, and locking seemed like a sensible extra perk for not too much money.
The mini keystone heads are small enough to not be cramped on a mandolin, the locking function works well, and the gearing is nice and low, and NEARLY as smooth as the top names on the market, but not quite and there is a tiny bit of gear backlash.
Overall, very happy with the purchase and the job they do. Fits the bill perfectly.
Now, what to do with the four tuners I have left over...