I know that I am a little late in this thread, but I wanted to share my Heavy Duty experience. I have made the best gains in my lifting using his methods. I gained nearly 25 pounds, (all natural from 190 to 215) ofcourse not all of that was muscle but my gains were phenomanal to say the least. I simply did not concern myself with losing size or strength and dedicated myself to his program and was rewarded for that belief.
However, I will say that using his methods will decrease your strength on some movements such as bench and shrugs most notably(they are not an active part of the routine). But, no big deal for me, my goal is to be big not the strongest man ever. My delts, triceps, legs and calves saw the biggest increases on this program. After nearly a year on the program I went to a 3 day a week schedule, much like the one Mentzer prescribes in his first Heavy Duty book, but with more volume to change things up on my body for awhile. My gains slowed down and I actually lost a little strength, but I have been quite active with my cardio as well, so that could be the main reason for this problem.
Now, i have started back on the Heavy Duty method, with some slight modifications to include more direct trap work and chest work during my workouts, and am already seeing changes for the better in my physique again. I would recommend Heavy Duty to anyone serious about making gains. If you like to hang out in the gym all the time and talk and play then this is not you routine. VERY INTENSE, VERY BRIEF, AND VERY EFFECTIVE. I trained several of my clients on this routine and they experienced twice the gains as my more "soft" clients. Mentzer was extemely bright and gifted and I give him credit for much of my success, but you have to read through some of his talk and get straight down to the training philosophy. He had his own set of beliefs for life and tried to incorporate that into his training philosophy, and that sometimes confuses people. In the end for me, I consider him a genius in his principles (as well as Yates, Jones, Viator, Ray Mentzer).
I recommend his books and training principles, but I do not advocate is maintance routine. Too little work and not plausible for the serious bodybuliding athlete. If any one has more direct questions for me I would be happy to answer them and explain his principles in more detail.