What's this thing actually do? [besides be an intersting choice for a temporary tatoo ?

It's a lowpass filter, but one which has four time constants all at the same frequency, and both gain and feedback. That means the following in terms of frequency response:

- A single R-C lowpass has a rolloff slope which declines at -6db per octave; that is, after the critical frequency, output halves for each doubling in frequency. This is not a terribly fast rolloff.

- Each additional R-C lowpass section you add increases the ultimate rolloff slope by another 6db per octave. But they load each other down and interact. You can do better by buffering each section from the others to eliminate the loading. This gets to being lots of circuitry.

- A lowpass with resonance (the gain and feedback give you this) can produce a peak right at the rolloff frequency which then dives for minus infinity after the peak. This sounds rather like a bandpass with a low pass added to it. Conceptually it is the same.

A lowpass withe peaking is kind of an ideal wah filter, as it lets through the fundamentals of the notes, but gives you the bandpass "wah" resonance on the harmonics in the human vocal range. The Vox wah is in fact a two pole lowpass filter with peaking.

A four pole lets you have sharper peaks, and the ladder setup lets you have an exponential control characteristic, which is important for musical sounding controls in a synth context.

It never hit me til now, but for effects use, I bet you get 90% of the advantage of the ladder by using only two poles, not four. That reduces the voltage requirements a bit and might make a decent 9V only circuit for guitar pedals. Hmmm. Have to work on that.