#1
i always hear about people saying how wonderful they sound. how do they differ from regular single coil pickups... assuming that P90s are single coil. are there any double-coil (humbucking) P90s? can they be split?
#2
I have a Gibson Les Paul Junior that has one single coil P90. It's definitely got a heavier tone over my strat with 3 single-coils. The distortion is pretty punchy and the clean is nice too. I haven't played any guitars with more than one P90 though.

I'm pretty sure that they do make humbucking P90s.
#4
They're basically a siglecoil that reads a wider area of the string, making them tonally about half-way between a singlecoil and humbucker. If you insist on using noiseless pickups you can get stacked, humbucking P90s as well though they're fairly specialist.
#6
They're like the best of both worlds effectivly. Fat distorted tones but you can dial in some nice bright clean tones too. P90s generally have a lot of Midrange punch to them.