This is my first review, so don't expect professional quality, but I had to work out my impressions of this show somehow.
I don't trust myself not to sprinkle minor spoilers throughout this review, so I'll provide a tl;dr. School Days is not what you expect, but that's OK, because it's better for it. It is more then it seems, and fans of psychological drama and twists should not pass it up. Avoid any spoilers or information and just go along for the ride. By contrast, if you expect a saccharine love comedy, go elsewhere. School Days will chew you up and spit you out.
I assume that if you read further, you watched the whole thing, and I'm not going to spend much time on plot summary.
When I watch any form of entertainment, movies, TV, etc., I may exclaim or yell at the screen once or twice, but rarely more then that. In the case of School Days, by the time things got rolling I was alternately yelling or muttering in stunned disbelief at least once, if not more, every episode. School Days had a knack for evoking a visceral reaction from me, more so than anything I've watched in a long time. It is not what you expect, and this defiance of expectations irritated, and still irritates, a lot of people. However, as far as I'm concerned, it is this very attribute that makes School Days one of the most engaging stories I've seen. It is not what you expect.
Going off of the first few episodes, one would expect a typical harem/romance story, with the protagonists experiencing their ups and downs before an ultimately happy endings. This IS NOT what School Days is. It IS NOT what you would expect if all you had to go off of was the title and/or a vague description of the first few episodes. Around the midway point, the tone shifts radically, as Makoto slowly becomes one of, if not the, most despicable and irredeemable characters in fiction as a whole. The decisions he makes and the consequences of those decisions are shocking.
A common criticism I've seen is that Makoto is just so...repulsive. he lacks a single positive quality and doesn't fit the mold of the well-meaning everyman that male anime protagonists are meant to be cast from. I agree, he is terrible, as are 99% of the main and supporting cast. Very few characters are without their own stupid or cruel moments. However, THAT'S OK. It is OK to have a work of fiction where the characters are despicable. It was actually a nice change of pace to have an anime where I didn't feel that every character was crafted for the sole purpose of getting the audience to like them. If anything, School Days swings things to the opposite extreme of the spectrum. The characters are so unlikeable, with a few exceptions, that it might have been detrimental in some ways.
This is not an anime that is going to have a happy ending, which becomes more and more evident as the house of cards that Makoto builds, the lies and relationships he slaps together, start to fall apart. As it becomes clear to Sekai that Makoto is not who she wants him to be, Makoto continues to degenerate, and Kotonoha becomes VERY unstable. I saw the ending coming, at least in part, although I did not expect the body count to be what it was. I assumed Kotonoha was going to go full yandere, but things shook out slightly different. The very tail end of the last episode, on the roof and on the boat, was the most legitimately shocking thing I've ever seen in anime. The boat sequence chilled me to the core. It takes quite a bit to shock me, but this, and the last half of the final episode in general, did so to excess. The guts it took to have an ending that bad in terms of outcome is staggering.
I've seen a lot of comments and reviews lambasting School Days for not being another by-the-numbers harem romance. From my perspective, far from being vilified, it should be applauded for this. It is undoubtedly shocking and I can understand the ending being controversial, but I would much rather have controversy than another squeaky-clean happy ending. If I had to choose one genre for School Days, it would be a psychological drama/thriller. It takes a cast of more or less unlikeable characters (Sekai and Kotonoha are the only ones in my eyes with redeemable qualities, possibly Setsuna) and allows them to develop the sort of messy relationships you don't often see in anime.
I doubt that I would ever want to watch this again. That does not mean I regret watching it, however. School Days evoked anger, pity, sorrow, disbelief, rage, rage again and some more rage, just for a change of pace. Rarely can a piece of fiction make me as angry as School Days did, and yet keep me watching. By the end, you know it's going to be horrible, but you have to see it happen, because without the catharsis of knowing that Makoto got some degree of comeuppance, excessive or not, then you're likely to explode.
School Days presents one of the darkest and unexpected story lines I've encountered in anime. It is one giant emotional rollercoaster crossed with a massive trainwreck. If you are a fan of psychological drama and of getting more then what you bargained for from a story, then this one you shouldn't pass up. I'll likely be analyzing and parsing my feeling on this one for a while. I might just have to go watch Clannad or Hanamaru Kindergarten, something overly sweet, to cleanse my palate.
Story (9 of 10): A complex, emotional rollercoster of a story. Very psychological, very dark. The ending is truely shocking.
Animation (9 of 10): The animation is crisp and appealing. The character design is good, and each of the main protagonists, at least on the female side, has their own distinct look. Makoto is your generic male protagonist, at least design-wise.
Sound (9 of 10): The voice cast does a fantastic job in their roles. Some of the secondary characters are annoying, especially the pack of girls that bullies Kotonoha, but that may have been deliberate. The soundtrack is good, although by the 4th or 5th episode the opener is out of place. It may have been better to replace it with a more somber OP.
Characters (8 of 10): The majority of the characters are unlikeable, but they are meant to be. Kotonoha, Sekai, and Setsuna have their redeemable moments, but the rest are by and large despicable. Still, with this sort of story, that's more of a positive quality then you would think.
Overall (9 of 10): School Days honestly surprised me. I had heard rumblings that it was not what it seemed, but avoided any big spoilers. I'm glad I did. The last anime to evoke such visceral, immediate, and lasting emotions out of me was Clannad. Of course, it was different with Clannad, and Clannad is in my eyes a vastly superior work, but School Days and Clannad are two very different beasts. One is geared towards a happy ending, one that provides catharsis through sadness and ultimately joy, while the other is all about the catharsis of rage, of bearing witness to a litany of awful acts and decisions and seeing things be resolved in a way that is simultaneously shocking, horrifying, and somehow satisfying in its own way.
The biggest endorsement I can give School Days is that, immediately after finishing it, I sat down and wrote this 1,200(ish) word review, regardless of the fact that is more than 5 years old by now. I did not enjoy watching it, but I am glad I did. I would love to see more expectation defying anime like this one. Next up, trying the game to see what the GOOD endings are like.