This is a weird choice of anime for me to watch willingly, because I'm usually not into romance-based storylines. The reason I did watch it, though, is because the reviews I read told that it wasn't your typical romance series, and it has one HELL of an ending.
I'm here to tell you that those reviews are right.
I was actually surprised how much I got into the storyline for the first half of the series. As I said, I don't normally like romance plots but I found this one to be particularly engaging. It's basically guy likes girl, girl likes him back, another second girl who is friends with both of them get them together but ends up liking the guy when its too late. Sounds typical and boring, but its actually rather compelling. Though I can't exactly say why.
It could possibly be because the lead characters are quite unique, and their characters actually develop and change over the course of the series. Makoto from a shy guy you don't really feel anything toward in the beginning (or maybe even somewhat relate to if you're a teenage guy like me), to an absolute ASSHOLE that you want to set on fire. Sekai goes from the confident, outspoken girl to the reserved and shy one. And Katsura, the initial girlfriend, probably has the biggest change of anyone in the series. Though I won't say anything here to avoid spoilers.
Most of the secondary characters are pretty one-dimensional and not interesting at all, with the possible exception of Setsuna (my personal favourite character of the series), who was the only one who seemed to show any kind of common sense. Katsura's bitchy classmates, instead of being interesting supporting characters that they could've been, were just infuriating bitches that served no purpose but to make everyone angry, so that's what brought the characters score down.
Apart from the storyline and characters, there's nothing else really to talk about. Animation was decent, music was appropriate, but nothing really stood out as spectacular.
And then wow, the ending...
Whether or not you enjoy School Days, you will NOT forget it in a hurry. Certainly quite a good series that I found to be a pleasant surprise.