Visual novel adaptations, at least of bishoujo games featuring routes for multiple girls, have been surprisingly rare as of late. The most recent such adaptations that I've watched go way back into Fall of last year, with White Album 2 and Little Busters! Refrain. Overall, the number of visual novel adaptations any given season seems to max out at two, with no more than five in any given year.
This is a bit jarring for me, as around the time I seriously started getting into anime (around 2007), visual novel adaptations were pretty much the standard non-manga anime adaptation source (kind of like how light novels are now). That, combined with the fact that many of my first anime around that time were of such visual novel adaptations, and my eternal love for cute animated girls, make visual novel adaptations among many of my favorites at the time.
I bring this up because next season has an abnormally large number of such visual novel adaptations. Sure, four might not seem like a lot, and wouldn't be if it were 2007, but now, especially after such a drought of them recently, it's definitely notable. These four are Grisaia no Kajitsu, Daitoshokan no Hitsujikai, Ushinawareta Mirai o Motomete, and, of course, Fate/stay night: Unlimited Blade Works, in case you forgot that was originally based on a visual novel (in fact, the original anime adaptation aired in 2006, right in the middle of the golden age of VN adaptations, though I hope this new adaptation is better than that one was...). Aside from Fate/stay night, I'm not familiar with these visual novels, though they all look interesting enough and I've heard good things about the original Grisaia no Kajitsu visual novel.
But since I don't have anything to say about what's coming up, I'll talk instead about some notable visual novel adaptations I've seen in the past.
Of course, when talking about visual novel adaptations, the big ones are the ones adapted from Key/Visual Arts works: Air, Kanon, Clannad, and more recently, Little Busters!. All four are favorites of mine, being well-crafted and emotionally powerful works that really make the most of adapting the medium (though some might contest that for Little Busters!).
Aside from the top-level works of Key/Visual Arts, there are some other VN adaptations that I enjoyed. Among my first was D.C. ~Da Capo~, part of a large franchise that has only recently appeared legally in the US when Da Capo III was streamed, though the original remains my favorite of the series. The first half isn't much to speak of, but once things get serious and some revelations about the nature of the eternally-blooming sakura tree that is at the center of the series are revealed, things get serious and even tragic, making for some great moments. That show is pretty much my baseline for what makes a good VN adaptation.
Some VN adaptations I enjoyed that are available for streaming currently on Crunchyroll are Myself;Yourself and H2O ~Footprints in the Sand~. I'll have more to say on those later, but feel free to check them out on your own in the meantime.
Visual novel adaptations tend to follow a pretty basic pattern. There are multiple girls, and one at a time, the show goes down each girl's "route", exploring some of the issues of each girl (and of the guy as well) and ultimately resolving them, either happily or tragically. Eventually, the show might settle on a "main girl" that will be the male lead's official love interest for the adaptation. Various shows might mix up the basic formula in various ways, but generally speaking, the quality of a visual novel adaptation lies not in originality but in execution: having a good, varied cast of girls to be emotionally invested in, and then writing out their story lines (and where applicable, romance) well.
Of course, with the saturation of VN adaptations during that time period, there were just as many stinkers as there were solid works. (The original Fate/stay night anime is a good example of such a stinker.) And going into the four shows coming up this season, there's definitely the chance that one or more of them will turn out to be pretty bad, too. Still, as a former (and in many ways, still a current) fan of visual novel adaptations, I am hoping that these shows can turn out to be good.