they aren't bad. i had one on a nashville power telecaster. one knob was master volume for the piezo, the other knob was a concentric and was volume and tone for the pickups. it was a little tall, but worked just fine. i can definitely see it working great for a guitar with tons of options. looks like a pain to wire up though haha
I use a lot of push-pulls, but don't like concentrics. Too clumsy, expensive, and you rarely ever need one if you lay out your pots right. On a two pot guitar I'll make one pot a volume for the neck and the other a volume for the bridge. When both are push-pulls, you can get coil cut for each PU separately, which gives you 8 PU combinations. If you EQ your PU's together with magnets & pot values, you have them set and don't need to adjust a tone pot.
I think they are great if you are rewiring a two control guitar for les paul style controls. I prefer 2 volume and 2 tone on my guitars and I'd rather drill out two small holes for coil splitting rather than not having a tone.