I saw Adaptation over the weekend, and at first, I agreed completely with your assessment of the film: great premise, great acting, weak ending. But then, I saw that the weak, or rather, conventional, ending, was on purpose - it was the part of the film that Charley's brother helped to write. That's why is was full of sex, guns, car chases, people learning life lessons - all the stuff Charley said explicitly at the beginning of the movie that he didn't want in there. It starts right after he sees Susan Orlean in the elevator and can't talk to her. He's gone as far as he can go, is completely stuck, and calls his brother for help. That's when he gets unstuck, at the price of making the film conventional and formulaic.

Charley and Douglas (who doesn't exist in real life, by the way) are living the movie they're writing. Not to get too deep, and I have no idea if this is what they meant,but we all write our own movies. In another age, they said that we all write our own books. But who has time to read, nowadays?