There are a lot of myths and theories in the training industry and one of them is that this system or that system is the ‘best’. To me, however, the very premise of a "system" is flawed. Each human body is unique and so is its exercise need. Once you try and fit a specific individual into a "system", you are missing the boat. Just a note here for clarity: there is ‘exercise’ and then there is ‘training’. Exercise comprises of things like walking, running, cycling, weight lifting and workout videos, while training is a specific set of progressions designed to get you to a specific goal.
P90X, Crossfit, and all of the other ‘exercise’ systems are not training. They are moving the body in order to get in better shape. Where the problem comes in is when we have non-experienced or under-qualified trainers implementing systems that are generic in nature and calling them personal training. Let's get this straight once and for all. If the program is not customized for the individual, it is guided exercise and nothing more. If your program did not start with a physical assessment and a fitness evaluation and, if it does not follow scheduled progressions according to your individual adaptation to that program, then it is not proper training.
There is no one system that fits everyone. There is no one miracle formula that, when followed, will fix everyone and everything. I am sure some “Crossfit is Life” people will jump up and down with rage when reading this, but it is what it is. It is a system, and, in my opinion, a very flawed one, but that is another debate.
This post is not designed to bash one or another "system", but instead to demonstrate that there is no one system that works for everyone. There can't be. All systems have their pros and cons. Some are worse than others, but they are all flawed in some way.
The basic principals of exercise are as follows: specificity, frequency, intensity, and duration. All of these principals need to be adjusted based on the individual and cannot be put into a "one system fits all" category.
If you are interested in exercise, that is fantastic and the best thing you can do for yourself. Be certain, though, to listen to your body and be sure that you are doing things the right way and within your limitations. Above all, if you are going to enlist the help of a fitness professional, then make sure you know their qualifications and ask smart questions. Do some research and go into your consultation with some knowledge of what to expect. In any business there are good and bad eggs. If there is one cure-all miracle system being pitched, then simply thank them for their time and keep shopping!