Rotary knobs provide greater control for smoother mixing imo. They definitely aren't for anything quick though. Good for mixing deep house and the like where the transitions are longer. That being said i still prefer using normal faders as it's what I'm used to and what most clubs use too. Pretty much what Rus said, they do provide greater control for smoother mixing. You don't want to try and do anything fast or fancy with it, but saying that I've seen a dude do some cuts and drops as well as other tricky shit with a Urei 1620 (original). Myself, I like to use it softly and take my time blending tracks together. I prefer to use rotary mixers, but I don't mind using faders. I definitely need to get an isolater for my mixer as I would like further control over the EQs but it's not a biggie, my records sound amazing through the mixer as is.