The original mode is glorious. It is FAT and rich sounding and will bring smile on your face. Put everything on 10 and dial your guitar volume accordingly. The guitar volume in this case will act also as a tone knob *just like Page used to tell you back in the day* . So that is sounding amazing even on the clean channel and can get from a slight overdrive up to face melting fuzz.
The MOD switch... Now here things get a bit weird. If you watched the promo video, the guy flips the switch and gets an even crazier gain from the pedal, however in reality, as you might see from other comments here too, the pedal loses all its bass frequencies thus is making everything much more quiet. So totally not like the video. If you put it on a clean amp, it will sound like crap. On everything at 10, it will feedback on itself, albeit it will still be more quiet than the original one. People have mentioned already that it sounds like a cocked wah, however that's not all. It actually sounds like it is on some kind of auto-wah AND on top of all the shittiness, it will eat your notes too - they will sound loud and nice for a split second, but then all of a sudden the signal will drop. Now I thought that this is actually defected (and maybe it is?), but judging by some other videos on youtube and other reviews here, it is common for all of them so far. So what is this mode for?
DIRTY AMP! Or a drive pedal after this one. It will sound insanely good and all the "defects" will open up and start to make sense. It sounds absolutely huge and glorious. It might be common sense for people who saw the originals, however the newer generation might not know that so it would have been amazing to include in the promo video or at least some kind of tutorial.
Anyway - a great pedal, even if you use only the original mode, which is why I got it in the first place.