My experience is that it does not affect the tone when it is off. The nothces in the sliders makes it easy to find the zero position. But they seems to be very sensitive. Meaning that a small change in slider position makes a rather big difference in the sound. Maby it is just my unit but it sort of reminds me of the EQ knobs on the BOSS MetalZone where a twist to far could really mess up your tone.
I use this pedal to brighten up the tone of my Ibanez RG Guitar wich, compared to my other guitars, has a rather dark tone. And in stead of creating a Ibanez patch on my BOSS GT1, I put this in front of the unit to EQ out the muddiness of that guitar. So once the pedal is set, I just turn it on and off and will most likely never touch the sliders again. The pedals small footprint on my board is also a good thing.
Can´t find anything bad to say about it.