Much of what is said in Free to Choose is essentially unassailable. Free markets work wonderfully well for many things, and governments are blunt instruments that often give the wrong incentives. One cannot argue.
However, there are a couple of areas where he pushes too far, into regions where it is not at all clear if free market capitalism will work as well as the book promises. Education doesn't simply fit into the free market mold because it's not clear who is buying the education. Is it the parent or the student? Is it the student before education or the student after education? It's also very different from buying shoes, because most people only buy one education; one doesn't get a chance to learn from failed purchases. So, it doesn't quite fit Adam Smith's model of two people, each rationally deciding if the transaction will be beneficial to him/her self.
Thus the book is missing something important: a discussion of how far one can apply the free market model. Friedman assumes it applies to everything, and maybe it does, but there are a lot of important cases where it's not at all clear.
But, really, I shouldn't complain too much. Every book has to stop somewhere, and there is much sense and very little confusion here. It's a readable book, and still relevant, even though history has moved on a bit.