I've read it. They have a simple point assignment convention used in conjuction with a limited collection of subjective judgements. It certainly doesn't exhaust all grading considerations or weights or styles of play. Its attraction is its simplicity.
We get, yada yada. It is simple, but a valid grading system to judge how a player played in a particular game. Not necessarily his skill compared to the rest. And honestly, its fine. Because guess what, the top dogs try and make their own rating systems that factor every aspect of everything, and it still gets scrutinized and ridiculed. And why? Because Football is very hard to quantify with Sabermetrics. The variables are so vast, and the amount of data available for each conceivable situation is so small, that it just cant be perfect. QBR and DVOA I think have great return for their numbers. Yet they are scrutinized constantly.
So lets get off the high-and-mighty horse and appreciate someone even taking the time to systematically record every down, for every player, in a timely manner and assign a grade. I watch the All-22, and rewatch games like a champ. Ive seen sundays game 4 times now, and have watched 5 others games as well. And I can tell you that I have not studied every single player on every single play. So I appreciate when someone takes the time too.