I despair when I look at some of the self-published books I've been sent for review. It's not a question of them having poor punctuation or questionable grammar: they're just awful, often to the point of being unreadable. The level of general literacy is embarrassingly low, and things like plotting and character development don't seem to have been considered.
I have a couple of these, actually. My trade-pub author friend receives vanity press books and manuscripts all the time, and then she sometimes sends them to me. To torture me, I believe. I'd describe most of them as delusional (and maybe a bit desperate) before I'd call them sad and despairing. I mean, do these people really think that my friend will read their ponderous tome of god-knows-what and immediately scamper off and tell her agent about it?
I've noticed two types of self-publishers. There's the aforementioned kind that get impatient/indignant over query rejections and go through a vanity press to become 'published' because they think they're just that amazing (or desperate). See: Dunning-Kruger effect. Or, they write and self-publish a single book that's actually pretty good, but then they sit and wallow in it, build a whole website just for this one book, make Youtube trailers for it, do all the promotions they can, whine about how it's not selling, and then ultimately refuse to play Amazon's game, which is to sit down and write more (Amazon tends to favor large catalogs and frequent, new releases).
Then there's the kind that have started to see it as a revenue stream, either for a supplemental or main source of income. Erotica is kind of its own beast in this regard - it's pretty much the one genre where you're probably better off self-publishing, but only
IF you're willing to essentially become a publisher for your work - which means thinking like a publisher (and not a writer) and taking care of cover art, distribution, and a crapton of market research all by your lonesome, and, probably most importantly, still taking your craft seriously and treat even your filthiest, most depraved pieces as lovingly as you do your 'real' work. I love all this. Many writers don't. Self-publishing isn't a good option for those who don't want to do the work on the publishing side of things.
So yeah, it's not for everyone, and it's not for every genre. I certainly don't think self-publication is the best course for my fantasy work, for instance. I'll be querying that out for reals. When it's finished.