Story
Koguresou Monogatari is basically made up of several related stories, each one following a different character. The reason why this isn't considered a collection is because the stories are related and even somewhat interconnected (with most of the characters appearing in each others' stories). Each of the stories give us a peek into the characters' lives, with some of them having romantic themes while others are more slice-of-life. I also like how the end of one story leads up to the next one. If I had one criticism, it was that most of the stories (and the manga overall) have an open ending (which is not nessisarily a bad thing, but is something to keep in mind if you prefer a more definite ending in the manga you read). Still, I found this manga to be a pleasant and enjoyable read.
Art
The art is probably the low point of this manga. While it's not bad per se, I have seen plenty of manga with better art than this one. Still, I suppose the art does get the job done (and I do like most of the characters' designs).
Characters
The cast is fairly vairied, with alot of them having their own quirks and/or troubles. Some are more likeable than others, but most of them do get at least some degree of development (though, some more so than others).
Overall
Overall, I found this manga to be enjoyable. If you like slice of life and/or manga that are made up of multiple interconnected stories, then you will probably like this one.