I know, it isn't specifically a pickup, but it interacts directly with and modifies the sound of one so... forgive me?
Pros:
Adds 'fatness' to every note.
Maintains core voice of the affected pickup.
Adds gobs of output and just a hair of compression (although I've heard from other folks that the output added is only to a certain point).
Cons:
Level of output can be too much if you're used to lower output pickups (or are more of a clean player).
There's a bit of high end loss that is likely the cause of a slight loss of clarity.
Adds in that 'active' feel to your sound.
Overall:
A fantastic idea that would be greatly improved with a bypass switch to easily go back to the normal voice of your pickup when desired.
