I installed this at the same time as a set of schaller locking tuners on a Peavey SC-2 that I'm using as a platform for upgrading. It was really easy to install, fits very easily.
I don't know if it's the tuners or the string butler, or probably a bit of both, but the guitar has massively improved sustain now. I mean it just sings.
Tuning stability has definitely improved, but again that is probably a combination of the string butler and the schaller machine heads. One point to note is that having the locking machine heads made it slightly less fiddly to install new strings. It's not very fiddly with the string butler, you just need to take your time if you're not used to it.
For the price and ease of installation, I would say this is a very worthwhile upgrade.