School me on filtertrons please

I don't know much about filtertron pickups, other than they look cool. What is it about their sound that makes them popular and what are there tonal characteristics?

me too

I have an old Gretsch, with one on there, it's a low output hummer with lots of clarity and chime. It sounds great clean and retains it's clarity with a bit of distortion.

They sound rather clear and bright for humbuckers - but they still sound like humbuckers, not like single coils at all.
You won't get any of that wiry, snappy twang on the low (wound) strings that a good Tele or Strat PU will give you, those strings will still ahve that woody, dull bark of a humbucker; the high (unwound) strings will have plenty of treble and bite - that combination sometimes makes Filtertron-equipped guitars soudn like two guitars in one: a SC guitar for the high strings, a HB guitar for the low ones. That works great for styles like rockabilly-ish Travis picking, but not so good if you are looking for ultimate low-string twang...
Another thing that's really typical of Filtertrons is their quality when played through a cranked amp (or an OD or distortion pedal): they always have a kind of grainy, crunchy, raunchy, gritty edge to the OD tone, they will never sound as smooth as a PAF-style humbucker...