It depends, if you're training with near maximal weights, and have a normal life, there is something to it. I do think most of it is mental, and the physical part isn't that big a deal, certainly not the "omg, I have the adrenal fatigue, and I'm hardgainer" type thing that some push. It's more of a mellow out and have an extra sandwich for a couple of days type problem.
As with most things, there's a time to grind it out, and a time to hang back.