HDII had me go from 165 to 190 in 4 months, with very little change in bf%.
However, I went from a sport of running around all of the time (tennis) to doing no cardio, as my collegiate career was finished, so objectively, not running around all of the time accounted for some of my natural weight gain.
I ate a very well balanced diet, including 2 Myoplex shakes a day (this was 1998). I also took Phosphagen HP, which was popular at the time. Eggs, milk, lots of nuts, bananas, apples, grapefruit, roast chicken, tuna, sushi, oatmeal was basically all that I ate, except for pizza on training day.
I started training once every 4 days, then went to once ever 5 days, where I made my best progress. After training sessions, due to reading/digesting everything ever written by Arthur Jones, I ate an entire pizza, as I envisioned myself as Sergio Oliva in 1972 (still to this day, I think the most impressive bodybuilder of any era, that year).
Many that knew me well, including my mother, accused me of juicing, as I had a half brother that was an obvious juicer.
Eventually, I became a knucklehead and forgot that I was bodybuilding, not powerlifting. This lead to piriformis syndrome, some back pain that resulted from muscular imbalance (fixing it still, but is a whole lot better), and not directly training arms (mistake).
Now, I am trying to get leaner, play golf, and get sleep (tough to do with a newborn).
Where I think Mentzer was truly off in his theories was that a pump was NOT necessary. It is not necessary for strength gains, but I feel it is necessary for changes in muscular appearance. This activity is all about illusion in the first place, no?