I definitely buy into it, just not as much as alot of HIT type guys make it out to be. I used to think training 2X a week with 4 exercises per workout and one set of each exercise was all that was needed. I learned the hard way that it doesn't do shit for growth, at least for me.
I train 4 days a week now with multiple exercises and sets and am making great gains. Bye bye to the 2 days a week crap.
I do notice my resting pulse is slightly elevated for a couple hours after training, but by the next morning it's back to normal.
Now if you train a set to absolute momentary msucular failure, with a static pause at failure, and then a superslow negative, then you will fry your CNS and take a week to recover. I know cuz I did this back in the day. What a stupid waste.
Bodybuilding and powerlfitng and any other form of training is about building a bigger, stronger body. It's not about annihilating it in the gym and feeling like a chemo patient for the rest of the week.
The best advice I ever got was reading an old article by George Jowett. It talked about leaving a little in the tank after each workout. Train until youre thoroughly worked, but still have some strength left. Don't train until you walk out of the gym all spent and shaky.