Makes it hard to definitively answer your question:
> Does anyone who is a long-form (novel) writer actually use Markdown?
I am aware of published authors who've used Ulysses, a Markdown-based multi-document editor for the Mac, for writing novels.[1] Semi-famously, mystery author David Hewson is a huge Ulysses fan, and wrote his own book about his process. I don't get the impression Hewson is super technically inclined the way, say, Matt Gemmell -- a software engineer turned novelist who's also a huge Ulysses fan -- clearly is. Some of the appeal for non-nerds is, I think, part of what grognards like George R.R. Martin and Robert Sawyer argue gives WordStar for DOS a big appeal: it's just you and your text with very little else to distract you. I've personally found that a bit overstated (most word processors have some kind of "draft mode" that, while perhaps not as minimal as just You And Your Plain Text, gets the job done), but it's clearly a thing, and I admit I enjoy writing in Ulysses more than I would expect.
Ulysses does, it's worth noting, have the capability to compile and export documents to Microsoft Word format. It's not as flexible (or overengineered) as Scrivener's compilation tools, but that's really something you should be saving until you have what you think is the final draft -- Ulysses and Scrivener, and for that matter NovelWriter, are ultimately composition tools, not editing tools. (Once you're in a "dialogue" with your editor sending Word documents with embedded revisions and comments, your manuscript is almost certainly going to stay in Word.)
I've written two novels with Scrivener, but I am slowly moving toward Ulysses for a variety of reasons -- but they are, indeed, technical reasons. I don't find the "detritus of special characters" to be particularly annoying with a well-chosen Ulysses theme; the underlines/asterisks are faded out and the italics and bold are, well, italics and bold, and for fiction that's virtually all I need.
> Outside of the more technically inclined...
Makes it hard to definitively answer your question:
> Does anyone who is a long-form (novel) writer actually use Markdown?
I am aware of published authors who've used Ulysses, a Markdown-based multi-document editor for the Mac, for writing novels.[1] Semi-famously, mystery author David Hewson is a huge Ulysses fan, and wrote his own book about his process. I don't get the impression Hewson is super technically inclined the way, say, Matt Gemmell -- a software engineer turned novelist who's also a huge Ulysses fan -- clearly is. Some of the appeal for non-nerds is, I think, part of what grognards like George R.R. Martin and Robert Sawyer argue gives WordStar for DOS a big appeal: it's just you and your text with very little else to distract you. I've personally found that a bit overstated (most word processors have some kind of "draft mode" that, while perhaps not as minimal as just You And Your Plain Text, gets the job done), but it's clearly a thing, and I admit I enjoy writing in Ulysses more than I would expect.
Ulysses does, it's worth noting, have the capability to compile and export documents to Microsoft Word format. It's not as flexible (or overengineered) as Scrivener's compilation tools, but that's really something you should be saving until you have what you think is the final draft -- Ulysses and Scrivener, and for that matter NovelWriter, are ultimately composition tools, not editing tools. (Once you're in a "dialogue" with your editor sending Word documents with embedded revisions and comments, your manuscript is almost certainly going to stay in Word.)
I've written two novels with Scrivener, but I am slowly moving toward Ulysses for a variety of reasons -- but they are, indeed, technical reasons. I don't find the "detritus of special characters" to be particularly annoying with a well-chosen Ulysses theme; the underlines/asterisks are faded out and the italics and bold are, well, italics and bold, and for fiction that's virtually all I need.