It is my own opinion that a "flatline" is beneficial, as we're seeing how far detached we were from the real image of sex and real people. It is only a real nuisance when someone wants to quit porn to immediately be chasing women around. For me, it has been a slow transition in my tastes - going from fetishes to real attractions. "regular women" are far more beautiful to me after being away from porn, and the stuff that really got me going before... well, it still attracts me, but not nearly as intensely.
Withdrawals included anxiety, depression, having some really dark moments play out (that normally would be ended by distracting myself with porn), and some serious ADHD sorts of things, which I am still battling. Physically, I'd get pain from not orgasming regularly. There was a really good thread on this recently, but I can't remember where....