I have only used it with EMGs but you CAN use it with passive pick ups but it will need a battery to run.
My experience is this:
It was a very subtle difference to the tone.
Not like a gain boost or mid punch but more like a slight EQ tweak about as dramatic as a regular tone knob only adding a mid hump and cutting barley any lows and highs.
I used it in a few guitars to try it out and uninstalled it from all of them. It is now sitting in my parts drawer for the last year.

What I ended up doing to make a KILLER mid/treble booster was this.
I took an EMG pa-2 gain boost (the same one that comes with the kerry king set), a simple resistor, capacitor, some shrink tubing and a mini toggle switch (all available at radio shack for under $10) and i built my own mid boost switch.
I can send you a diagram if you like
Its simple and sounds awesome.
It rolls of the low muddy sound of the pick up and boosts the highs and upper mids so when you use your neck pick up it turns into to a soloing machine.
Every note is right up front and free from the muddy bottom end tone. also it gives you an output boost and more sustain.