I should probably note any of the inherent biases that I have here. When talking about Howard and Conan, there's a sort of continuum of fans, and you can plot out fans on that continuum based on what appeals to them. On one end, you have a Howard fan who is a "purist", for want of a better term. He or she might acknowledge only the works that were directly written by Howard, and does not follow the movies, comics, pastiches by other authors, and so on. At the other end, we might have a person who is really a fan of the Conan character specifically and follows everything related to the character. Of course, I can't speak for everyone, but this has been my experience.
I like to compare this to the fans of two other long-running iconic characters, Sherlock Holmes and James Bond. There are Holmes fans who acknowledge only the Doyle canon, and other fans who also enjoy the pastiche work like other authors' books, the movies and the Jeremy Brett TV series. Likewise, we see fans of Ian Fleming's original Bond stories, then there are the fans of only the Bond movies, which are of course different from Fleming's books. One type of fan isn't better than another, and I'd like to think that all the fans can find at least some bit of common ground.
"Civilized men are more discourteous than savages because they know they can be impolite without having their skulls split, as a general thing." --"The Tower of the Elephant", 1933.
readily available in print (as is the case with the marvelous work by Howard scholars for Wandering Star and Del Rey), then there is plenty of room for expansion of these characters' adventures. In my opinion, L. Sprague de Camp and Lin Carter's work on the Lancer/Ace series (and those Frazetta covers!) was very important for making Conan -- and by extension, Robert E. Howard -- popular with a wider audience in the late 1960s and the 1970s, and should be recognized for that accomplishment. Even so, the integration of their own writing into the Conan "mythos" and their rather brutal editing of the originals are difficult to forgive. I hate to even bring up such contentious work like Dark Valley Destiny, de Camp's psychological biography of Howard that introduced some erroneous facts and outright baseless conjecture. Scholars are still trying to repair lasting damage to Howard's reputation 40 years after the Lancer series first appeared.
There are great stories in some of the comics and pastiche novels. The novels tend to be hit-or-miss, depending on the author. Some are great (John C. Hocking, Karl Edward Wagner), others not so much (Leonard Carpenter, Poul Anderson, Steve Perry). The movies, for the most part, are fun and have some cool parts in them -- more on that in a later post. If you exclude some of these works out of hand, you'll miss out on some really good entertainment.