The preamp is very easy to use, one knob, 4 buttons, everything is simple and clear.
I didn't used it with any microphones yet, only with my Axe Fx II. I placed it between Axe's OUT and audio interface (Focusrite Forte) IN.
As expected API added some punch and sparkle to guitars sound, both clean and distorted and somehow colored the tone making it more musical and less digital. The preamp can be easily driven up to +6 db and even more without audible distortion. A little high frequency crunch may appear but it sounds like a feature.
I would highly recommend this preamp to everyone who messes with digital stuff. This thing can definitely breath some life into digital sound.