So there are three different sections to this book, all quite different from each other.
The first section is a short biography, which covers Stallone's life from birth to the time this book was written (around 2005). It's more of an over-view than an in-depth account, but this is easily the most readable part of the book. He's led quite an interesting life, and I found it quite easy to read and become engrossed in despite not being a fan of his work. (Not that I dislike it, I've just never seen any of his films.)
I'd imagine this section would be even more interesting to those who are fans - he mentions a fair number of interesting anecdotes about working on his films, the though processes that went into making them, and what methods he used to get into shape for the various roles (not to mention the various processes he tried that most emphatically didn't work - the yoghurt diet is one that will stick out in my mind for a while).
The second section is a bit harder to get through in some ways, despite the brevity of text. This section mostly describes various exercises that he recommends for those looking to get in shape. Interspersed are various tips on performing the exercises, a few examples of exercise plans a reader could use, as well as tips on how prepare and psych yourself up for working out, etc. It's definitely a useful section, if not as read-able as the first. The only issue is while there are descriptions and photos of the exercises, it would be helpful if there were a diagram of some sort to show you positioning and such. Or a dvd included, perhaps, to walk you through some of the exercises.
He does recommend a few times getting a trainer, however, and mentions that most gyms offer introductory sessions. As several of the exercises he recommends require gym equipment, it's likely expected that practical instruction would be offered by staff there.
The third section of the book covers diet & nutrition. He discusses various diets he's tried over the years, and the various positives and drawbacks he found them to have. He doesn't diss all of them, insisting readers follow his methods - he's pretty neutral towards many of them, notably weight watchers, though he does acknowledge that it may not work for everyone.
A lot of what he says, though, is mostly common sense that we've heard before - lots of vegetables, healthy meats, whole wheat over white flours, etc. He talks about what works for him, and how to stay on track with healthy eating. This section also has an anecdote from his movie days which had me giggling, so even if you find the subject matter a bit dry, I'd still recommend at least skimming through the section. If nothing else, the information is presented in a very encouraging manner which if you're feeling stuck or in a rut might be the encouragement you need to get back on track.
One thing he recommends a few times is various protein powders, and 'protein pudding' which I hadn't heard of before. Again, this isn't a new thing - I've heard before how important it is to make sure you're getting enough protein when working out to make sure you can build up muscle - it seems like less of an alien idea with the way he presents it, however. A protein shake seems a lot more manageable than a story I heard second hand about downing entire chicken breasts as a snack after a work out, for instance. And Stallone's recipe for oatmeal cookies sound a lot more appetizing (though personally I'd look at adding dried fruit, perhaps - playing with the recipe a bit).
He recommends supplements like flax seed oil as well, but not a lot of information is given, so personally I'd probably look into them more before making any decisions there.
There are also a few small after notes to readers at the end of the book - ways he finds to encourage his family to be active and healthy, an example of his daily schedule, etc.
One thing people complain about often is that he does make a couple of name-drops throughout the text. Personally, I didn't find it annoying or off-putting in this case, but others may differ.
He doesn't do so frequently, and there was only the one or two points where it seemed out of place. It would be stranger to me if a celebrity writing about their life didn't make some mention of other celebrities, especially those still working in the industry - they don't live in a bubble after all. Of course he mentions various people he's worked with over the years - when discussing his films, for instance.
It seems like a good reference for people considering branching out in their exercise routines, especially if it's true what he says about cardio not being a fix-all solution (I have no reason to doubt it, and his arguments make sense - I don't know enough about the subject to say for sure myself though). Personally, I'd like to see him come out with a longer, more in-depth biography, though. That was the most readable section of the book, and I found it really interesting.