If you've got some basic or advanced knowledge in any of the areas Null covers, you can skip past or skim that particular section without it detracting from the balance of the book. It's likely, though, that his discussion will at least reinforce important points. Moreover, the book is never academic or stilted. To the contrary, he comes across like he's working with you one-on-one at the kitchen table.
Despite Null's warnings, the book does carry the potential for creating the impression the world can always use more film critics. Still, whether you become a published film critic or simply one in your own mind, Null's discussion of film theory and styles, and his personal list of 300 "Must-See Films for the Aspiring Critic" are alone worth the price of admission. I guarantee you will come away watching movies with a different – and more informed – perspective.
But, like I said, keep this on the down-low. There's already enough competition out there for those high-paying reviewing jobs.