I'm a voracious reader of traditional books. In books and e-books I normally read everything (except in case of technical literature). Occasionally I skip entire paragraphs, but this is really a sign that the author did something wrong.
For the first reading of a visual novel essentially the same applies, but the average quality of the writing is lower and therefore it happens far more often that I skip a lot of boring text. Typical offenders include lengthy introductions, "tell, don't show" sections, and flashbacks that are simply too mundane to be interesting. Boring passages are sometimes accompanied by very little change in the accompanying illustrations, and occasionally even by a black screen. I also skip text when going through the same passage again soon after having read it (e.g. because I reached a bad ending). A third reason to skip is for the equivalent of browsing through a book in a bookstore.
Skipping is such a regular activity even in excellent Ren'Py novels I always start by activating the preferences options "Skip unseen text" and "Skip transitions" for maximal skipping efficiency. By the way, it's always annoying if a sandbox-style passage that serves no real purpose gets in the way of skipping with the Ctrl key. (Sandbox-style controls make sense for explicit scenes where you want to give a choice of what to watch next, and for RPG-style minigames -- hopefully not too grindy! -- such as the ghost hunt with Sasha in Stormside or a party in Being a DIK. They also make sense in management games such as Sexbot Restoration 2124 or in trainer games. But in slightly over half of all cases a sandbox-style Ren'Py game would be improved significantly by converting it to regular visual novel style.)
Despite being a reader of regular books as well, when reading a visual novel I am not too picky about the text. It's the same as with a comic book. Great text can raise a comic book (or visual novel) from good to excellent, and atrocious text can pull it down to mediocre even if the images are outstanding. But the images definitely have more impact than the text. Although certainly not ideal at all, with some exceptions I would prefer navigating through a great visual novel such as Eternum or Cosy Cafe seeing only the images and no text, rather than the other way round. Just like I used to peruse comic books before I could read.
I imagine that for most authors, getting the images right is the main problem. Most readers forgive awkward text so long as it's not so blatant it becomes annoying (e.g. hard to read due to Engrish or because of exCESiff orfografic orr krammar plunders), but are less forgiving when it comes to the images. In fact, you can't make everyone happy with the images. Some readers will reject every novel that uses Honey Select, or even any familiar character model. (I wonder how they would react to seeing a famous actor in a movie after having seen them previously in a different role...) Some will be picky about minutiae of posture and facial expressions, while others tolerate comically stiff and schematic positioning such as the sleeping positions in
Jeijei's Harem. (In that game the text is similarly eccentric. For me as a seeker of the unusual, both features actually paradoxically increase the enjoyment. In connection with the astonishingly precise portrayal of Jeijei's programming job, it feels to me like being allowed to share the world of someone far on the autism spectrum. It's a rare privilege.)
That said, once the images have reached a certain level of quality it might be a good idea to get high-quality feedback on your text. While it has less impact, it's also much less effort to optimize it.