I have a question about your suggestion for using an engineer bag stuffed with weights as resistance training, though [...] but I really wonder how favorably it compares to pumping iron with real free weight equipment.
I should preface by stating that the engineer bag (or other suitably heavy object you might train with) is not a short-cut or temporary fix to get people started with resistance training; it is a fully-formed method suitable for people of virtually any level of fitness, and can be an effective program for life. The barbell is less than a century old; it’s not like prior to that humans were just wads of cookie dough with stick arms.
A lot of ‘underground’ fitness methods do Strongman-style training, with tire-flipping, sandbags, car pushing, stone lifting, and so on. For all intent and purpose, you can build an incredible physique and immense strength doing basic movements with challenging free-weight of any kind. It’s just mass being moved, simple as that.
That said, it would depend on your goals and how you define them.
Bodybuilders, to start off with (and pardon me if I paraphrase something you might have already read, I say a lot of things more than once) are particularly suited to using barbells and dumbbells because what they are doing is in essence being sculptors, with their own body as the medium. Even amateur-level bodybuilders spend hours just critiquing their own physique, and by having a wide range of tools and stimuli available they are able to shape each curve, angle and plane. In that case, not only would you want the more refined tools that a gym offers, but a host of other isolation exercises that I don’t get into in the book.
Powerlifters and other competitive weightlifters I consider to be athletes, and lifting barbells as a sport. From the lowly gymrat up to the girthiest (sure that’s a word shut up) of pro powerlifters, they seek to make the numbers go higher, same as a runner wants to make the numbers go lower. In this case, I would recommend you seek a gym as well; it’s significantly easier to lift 225 on a bar, even in a front squat, than it is to do the same with an engineer bag or any other unwieldy load. This isn’t to say it’s somehow less of an accomplishment, but like any other sport, some powerlifters wear special suits and practice special techniques (and often have horrifying physiques and form) just to get the numbers a little higher. It’s not a means to an end, it’s the end itself.
There are obviously other purposes that I may not need to get into for the moment, but suffice to say if you goal is “be leaner”, “be stronger”, “look good naked” or most of the reasons people want to ‘do exercise’, the engineer bag is not only a valid choice, it’s one of the better ones available. Though I personally love the gym in theory and in practice, and consider my power cage to be a part of the family, it’s not necessary. I (and you) could accomplish basically the same thing with a good sized rock.