I think Herbert did, to some extent, plan the first three from the beginning, but I can't recall exactly where I read that. IMHO anyone who really cares about the first one should at least read the second to see what he was really trying to do. That said Analog Magazine, which published the serials that were combined and expanded into the first Dune, wouldn't buy Messiah. So that's a pretty telling example of people being on board with the original and then not liking the turn he took in the second one.
Despite the fact that first one is one of my favorite SF novels and I liked the next two as well I haven't gotten around to the rest. I know BD liked God Emperor and I own it so I'll read it at some point. I just don't have completest impulses when it comes to SF series' since there are so many long running series that are reputed to turn into cash-ins after a while.
I also like Lovecraft alot but have never read through his complete works. The major story, at least in terms of its length, that I did not finish was The Dream-Quest of Unknown Kadath, which was too gobbledygook-ish even for me. And I didn't bother reading all of his earlier, shorter workers because the ones I read were alot weaker than his more well known stories. But I'm a big fan of the core of his cannon.
The history of Lovecraft's popularity is kind of interesting. His friend August Derleth, who founded Arkham House publishing company, claimed Lovecraft had handed him the copyright for all of his work. Derleth and Arkham House managed to keep him in print and publish new "Cthulhu Mythos" stories by other authors, keeping Lovecraft's reputation alive. Then they made a killing off of licensing his work to Chaosium, a gaming company. That allowed them to put even more money into their Lovecraft publications, preparing good critical editions of his work. And then in the 90s Chaosium demanded that Arkham House prove they actually owned the copyrights that they'd been collecting royalties on for decades, and it turned out that they couldn't offer anything to substantiate their copyright claim! I think that's what lead to the explosion of jokey Cthulhu merchandise that seems to have become more popular than his actual stories.