Orson Welles directed eleven movies during his lifetime. His movie-making career spanned more than thirty years, and eleven movies isn't very many movies to make in thirty years, but he had quite a few difficulties, only some of them of his own making. In addition, most of his movies were not released in exactly the form he intended, and sometimes they were even edited without his participation.
The common idea many people have about Welles is that he peaked early and then fizzled out. After all, Citizen Kane was recently cited as the best American movie ever (which it isn't) and it was the first movie he ever directed, at the age of 24. This is easy to think, though, since both Citizen Kane and The Magnificent Ambersons (his second movie) are clearly great films, and after that much of his work was done outside the traditional studio system, and often outside the United States.
However, even though his later movies were usually made for almost no money, often under very difficult conditions, they are all worth seeing, and several of them are as good as any movies ever made by anybody.
Few people except real film buffs (and probably not that many of them) have seen all of Welles' films in theaters. They (the films, not the film buffs) are very hard to see, many of them are not easily available even on video, but I have seen them all (in theaters), since I was lucky enough to live in New York during the golden age of the movie revival houses, before video killed them off. Some of the movies I haven't seen in many years, however, so I have used some reference materials in order to make sure I don't go completely off the tracks.
Oh, about Citizen Kane specifically? If you haven't seen it, go and see it immediately. If you have seen it, go see it again. Virtually everything you've ever heard about it is true.
Later: If you have seen it several times, get the DVD with the commentary track by Roger Ebert. You'll learn a lot.
"If I had learned to compromise more, I might have made better films, but they wouldn't have been my films."
(Orson Welles, when he was given the American Filmmakers Institute Lifetime Achievement Award – quoted from memory)
"This is the biggest electric train set a boy ever had!"
(Orson Welles, referring to a movie studio)
"I don't love films. I love making films."