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?
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.
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.
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.