I have two sets of P90 pickups. The first set I have are in my 1968 Gibson SG. They are raunchy to say the least. I think the hype about them stems from the fact that they were the original pickups in most Gibson guitars. They are much larger than the fender singles and they have a pretty full tone. Along with that comes the feedback and the 60 cycle hum.
If you are one that is not a fan of unwanted noise from your guitars you will NOT like them. If you don't care and want some raunchy, over the top tones they are for you. Most of them are hum cancelling in the middle postion with the volume controls equal. My other guitar has a more modern set of P90s in it. The different in the output is substantial. The old p90s are in the 7.5k range and the new ones are 12 to 14 range. Lots of noise as well but hum cancelling in the middle.
You can get hum bucking p90s, but that is why they made humbucking pickups in the first place.
I am a huge fan of humbuckers and second would be the p90s.
If you can listen to sound clips or even try one out, make sure you listen fro the noise, and or try it along with a humbucking guitar.
If you know anything about Fender style singles you know they are hum cancelling in position 2 and 4. These are twice as big and only hum cnacel in the middle postion.