His work exploring politics, the drugs culture, the Hells Angels, etc. was enormously influential and widely parodied in all genres - see, for example, Games Workshop's Judge Dredd RPG companion in the 1980s, lots of SF including the first Wild Cards book, etc. - and started the modern era of journalism, in which the journalists reaction to events have become part of the story. For my own part I admired his work, parodied the opening to Fear and Loathing in Las Vegas at least twice, in fanfic and in games reviews, and probably haven't read nearly as much as I should.
He'll be greatly missed.