For those not in the know about such things, back in the late '60s/early '70s, L. Sprague DeCamp worked out a chronological timeline for Conan's life and career and repackaged altered versions of Howard's stories, along with other Howard stories re-written by DeCamp as Conan stories, Howard fragments expanded to full stories, DeCamp stories written from unused Howard outlines, or all-new (at the time) stories written by DeCamp or Carter. DeCamp referred to it as "posthumous collaboration"; less charitable commentators referred to it as "grave robbing" or other similar things.
DeCamp is a problematic figure for Conan fans; the Lancer series was a big hit and brought about a huge surge in Robert E. Howard's popularity. It was instrumental in Marvel comics' production of their Conan the Barbarian series, which really brought Conan into the popular culture. Many if not most of us who know Conan today, got our start by reading the DeCamp version. So to that extent, we're grateful.
On the other hand, DeCamp's stuff, well, just isn't as good as Howard's. Imagine if Salieri got ahold of the rights to Mozart's work and published it after making all kinds of random, fiddly little changes and mixing it up with stuff of his own, and you might have some idea what I'm getting at. Salieri was a perfectly serviceable (if unspectacular) composer ... DeCamp was a perfectly serviceable (if unspectacular) writer of sword-and-sorcery fiction.
After having spent a few years immersing myself in Howard purism (so to speak), the non-Howard pieces now stick out at me generally, and are occasionally quite jarring. Without having the texts at hand it's kind of hard to give concrete examples, but I can generalize a few things that I noticed:
"Conan" Meets "The Hulk" -- Howard's Conan was certainly larger-than-life: tall, hard-bodied, imposing, determined. While uneducated, he was a fast learner and a natural leader. The deprivations of youth in Cimmeria had made him naturally resilient, self-reliant, and athletic. DeCamp's Conan was all of those things plus master of every weapon, a marathon runner, a deadeye shot, able to ignore all but the most crippling of wounds, and so on.
Arcanaphobia -- Howard's Conan dislikes wizards for the most pragmatic of reasons ... they're unpredictable and chock full o' weirdness! At the end of "The Scarlet Citadel," when the Pelias-bird goes flying off with Tsotha's head, and Tsotha's body goes shambling across the desert after it, Conan essentially says, "What the crap was that about? Gah, I need a drink!" A perfectly reasonable view, I'd say. Conan even does some sorcery of his own in "Beyond Thunder River".
DeCamp's Conan, on the other hand, can't so much as meet a fortune teller without getting the willies crawl up and down his spine. He's forever getting creeped out, frightened out of his wits, or generally acting like a '60s sitcom housewife who sees a mouse whenever a wizard pops up. Is this DeCamp's attempt to throw some kryptonite at his Cimmerian Superman, or is it a hamfisted way to make sorcery seem frightening? Impossible to know.
On the other hand, one thing I rather liked about the DeCamp stories was that they were a little freer with the fantastic. Howard thought of the Conan stories as being largely "realistic" within the "mythic prehistory" context, and so when monsters appeared they were usually either outsize versions of existing animals (thirty-foot snakes, ferocious killer apes) or extraterrestrial things that slithered down from space.
DeCamp was more comfortable in simply making a monster, a monster. Where Howard's Conan would face a Komodo dragon the size of a Volkswagon, DeCamp's Conan would face an acid-spitting weird-music-trilling slug-thing the size of a schoolbus. A bit more colorful. :)
Somehow -- The laziest writing technique evar is "how he escaped from that den of murderers, he was never quite sure" or "somehow he broke free". What it basically means is that the writer either couldn't or didn't want to think of how the hero achieved whatever needed to be achieved, and so had it all happen in a frantic haze of luck. The DeCamp/Carter duo, unfortunately, fall back on this technique multiple times. For an unstoppable juggernaut like their Super-Conan particularly, there's no need to wonder how he got out from under a pile of foes -- he just flexed his pecs at them and they exploded, obviously!
Um ... Cimmerians Don't Go to Paradise, Dude -- I don't think DeCamp ever got the point of Crom and the Cimmerian religion; he seemed to think it was like Valhalla, a shining hall with warriors fighting all day and carousing all night. You certainly wouldn't pray to Crom, or else he'd be like the abusive father who says "Oh, you want me to really give you something to cry about?"
Cimmeria was a bleak, gray country of perpetual twilight caused by a) high latitude causing the sun to always be low in the sky, and b) hilly and barren terrain that blocked what direct sunlight they might get. Cimmerians had a cold, hard, dark life and appropriately cold, hard, dark gods to go with it. I remember reading that the Cimmerian view of the afterlife basically consisted of wandering around a foggy twilight underworld until your ghost faded away; I can't remember now if it was actually Howard who said that or a later writer (possibly Vincent Darlage), but it's certainly closer to what I would expect of the Cimmerian religion. The one time in Howard's work that somebody mentions having met other Cimmerians besides Conan, his comment is that they never smile and only drink water. These people would carouse in Valhalla? I don't think so. Conan's boisterous nature was the exception, not the rule!
I can see now why the Howard purists tend to be so vehement; imagine if Harry Potter were repackaged somewhat out-of-order and combined with what boils down to "better than average fanfic" to fill in the gaps and you'll perhaps see where they're coming from. Combine this with some copyright-law-shenanigans after Rowling's death that makes it look like you're essentially trying to co-opt the rights yourself (whether you are or not) and you'd get a pretty furious fandom.
On the other hand, if the pastiches and fragment-rewrites had been published on their own with a big "based on the work of R. E. Howard" across the top, they would probably be honored and welcomed additions to Conan lore, lesser quality notwithstanding. Nobody can write like R. E. Howard but R. E. Howard, and very few people would expect you to ... it's the mixing of DeCamp/Carter's work in with (and alterations of) Howard's originals that really makes the Howard purist's blood boil, I suspect, as it implies an attitude of "my stuff is just as good as Howard's, if not better". Neither Roy Thomas nor Cary Nord have been branded with the kind of viciousness as DeCamp has, despite having taken Conan further afield from Howard's work than DeCamp did (not to mention doing the exact same "repackaging other Howard stories as Conan vehicles"), because neither of them ever claimed to be doing anything but loosely adapting the character for their own purposes.
Did DeCamp actually have that attitude? I certainly don't know and since he died in 2000 I'm not likely to find out. Honestly, I don't think he did, otherwise he would have stuck to writing his own work instead of trying to co-opt Howard's. The fact that he did most of his fiction with collaborators (whether living or "posthumous") suggests to me that he was reasonably good at prose but had trouble with story -- of course that may just be me projecting my own issues on him. ;)
But now that Howard's original works are so readily available, I wouldn't mind seeing the DeCamp/Carter pieces brushed up and republished in their own volumes with a clear explanation of what they are, where they came from, and why they're significant. If you go into them looking at them as "better than average fanfics" rather than thinking you're getting "the real Conan" (so to speak), they are very enjoyable.
Except when Conan doesn't know how he escaped. ;)