Fear and Loathing in Las Vegas is the exact kind of narrative that I don't think works well as a movie. The story is so profoundly introverted that an almost purely extroverted medium like film cannot possibly do it justice. I found it coming off as awkward and silly at times through no fault of its own: simply because it was a film trying to express something that film is not well equipped to express. It was certainly an interesting experience, and parts of it really did work out well (such as entering Bat Country), but it largely fell flat for me, whereas the novel did not.
Also think impersonating Hunter S. Thompson affected Johnny Depp so much that he never left it behind. He uses it everywhere. I can see touches of Thompson in everything he does now!