I read books by his predecessor Syd Field. I think they are useful to get a basic sense of storytelling when you start writing but over time you realize its also rather obvious advice --anything can be presented as having 3 acts if you search for it. If you write a story you want it to have certain things- plot, character, drama, interesting turn of events, etc.
I have only heard McKee talk in bits-but he is probably right that the best writers in the US are doing cable tv shows, not feature films (how many original screenplay-based movies are made these days?)--and many of the current movies I see don't seem to follow basic screenwriting (or storytelling) structure either.
I wouldnt want all films to follow a US storytelling model though, but I think it is good when used with some motivation. A movie like Clash of the Titans 2010 might use a 3 story arc but if so its by accident.