It is a way of simplifying the making of exposure and development decisions that will give you negatives you need to get prints you want. Ralph is correct: It is horrendously simple. That is the whole point of it, IMO. If it ceases to be a simplification, forget it. If you are confused in any way by it, forget it. It is not for you, and there are many other ways to work that will let you do just fine.
I know how to use it....quite well, in fact. I still use it sometimes, and used to use it a lot. However, I do not hold it up on a pedestal. My problem with it is not technical, but "conceptual", you might say. My problem lies in the initial testing for "normality". The way I see it, it is not the testing of what a film's natural characteristics are so that they can be harnessed and used artistically. IMO, it forces films into a pre-defined box as to what "normality" is. The initial procedures take every film, and tweaks it into the same basic film, in terms of rough contrast. In Zone System methods, every film is manipulated into behaving a certain way from the start. All films are forced into the same mold with the initial testing procedures. All films are tweaked such that, when printed, they provide a certain low-toned and high-toned value at a certain amount of exposure and development, respectively. As such, I feel that Zone System methods take away much of the individual characteristics of films, instead of just letting them be what they are naturally, and learning to work with them. I feel that learning each film inside and out, metering for a midtone (incident metering), and judging the brightness range of the composition in order to make exposure and development tweaks works faster, is far less prone to "operator error", and in 90% of cases, gives the same exact exposure and development that your basic Zone System methods would call for. The difference is that the incident method requires understanding and being able to judge light to get what you want, while with the Zone System method, the spot meter hands you everything you need to know on a silver platter, and you specifically decide what tones certain elements of the composition will be. For the 10% of situations in which I know an incident meter will not be the ideal tool to give me what I want, I use the straight Zone System.
Personally, when I have time, I use a combination of both methods. First things first: I calibrate my incident meter to my spot meter. Next, I take an incident reading and get my "base" exposure. Then, I set the spot meter to this exposure. This gives me an EV that lines up with the red dash. Then, I meter whatever luminance values within the composition that I want to measure. When I measure them, I am looking at how far from the "base" EV they are. Since I already know how my film behaves, I know what sort of a range it will capture at the "base" exposure, and what it will not. If it will not capture what I want, I manipulate exposure and development so that it will. This is so quick, and even if misjudgments are made, there is a printable negative anyhow, because the base reading was for middle grey (incident). IMO, straight tonal placement with a spot meter alone is far more error prone and has a steeper learning curve.