I read (and loved) The Blind Side a few years ago. It's a great piece of sports writing with a great story in the middle of it. That story makes up the core of the movie adaptation.
And generally, the movie stays pretty true to what actually happened. The biggest change from the book, and the hardest for me to get past, is the focus on the adopting mother (Sandra Bullock) as opposed to Michael. This is like watching the story of Hamlet through Gertrude's eyes–the most interesting character is not the focus.
Even so, it's an affecting story and pretty well-served by the movie. If it makes you pick up the book, then by all means go see it!