I read a lot of Mills' stuff maybe 7-8 years ago when he was marketing through his free newsletters and also touting his son's great success as proof positive of his theories.
I think you can learn some from him, but you can't take everything he says as gospel. Some here are prepared to reject everything he says, though, and that's not right either.
In particular, Mills completely convinced me that working on pushing off the rubber was wrong, and that mastering an explosive hip/trunk turn was the key to both building velocity and command. I also learned a lot about proper arm action from reading Mills' stuff.
But I never bought the whole Mills program. For one thing, the man's basic concepts would fit into a single magazine article, if only he would submit himself to an editor. Instead, he repeats himself so often and then goes off on tangents and what have you so much, that you really do need a highlighter pen to go through what he writes to get out the few nuggets worth remembering. The rest could hit the cutting room floor.
I also think it's wrong to suggest that there is only one way to succeed in baseball, whether we're talking about hitting, pitching, or whatever. Case in point: How many would have taught a young Tim Lincecum to throw the way he does? Maybe NOW we would, but be honest, who would've taught that throwing motion to their sons 10 years ago?
Time has also shown that no single philosophy has all the answers. Mills' son was a lefty whose velocity got into the mid-90's at times. My understanding (correct me if I'm wrong) is that he struggled in the minors and ultimately fell victim to recurring injuries. Baseball can be a cruel mistress. The last time I read anything by Mills, all the stuff about his son was no longer part of the spiel.
If he's now arguing against long toss, that's a development since the days when I read his stuff. And based on my experience, abandoning long toss is very bad advice.