A lot of people don't like it because they muck with his character and make up some kind of fairy story about him being a pacifist trying to stop the war, who regularly lands his plane in No-Man's-Land for cosy, chummy chats with his best pal Roy Brown.
Personally, I wouldn't care how they messed with history if it made for an entertaining film. But the crazy thing is, instead they made the most turgid, boring, flat piece of poo imaginable. The real story is much more interesting. You just don't care about any of them, all you want is for the film to end. It is so boring.
The flying sequences are the best I have seen in any film, but they are so short it is not worth suffering through the rest of it for them. Find them on Youtube, instead.
For all its faults, Flyboys is a much better film. It at least fulfils the minimum requirements for a film: It entertains.