The Disappearance of Nagato Yuki-chan exists on two levels, which makes it hard to review:
1) As a Haruhi Suzumiya spin-off. On this level, it mostly fails.
2) As a stand-alone, slice-of-life romantic comedy. On this level it mostly succeeds.
Kyon joins the literature club, along with the shy Nagato and her best friend/surrogate mother Asakura, thereby saving it. Nagato quickly falls for Kyon, and Asakura tries to help her build confidence to confess. Meanwhile they have stereotypical manga/anime antics (Christmas, Valentines, Tanabata, Fireworks festival, Onsen trip, beach trip, etc.) with Haruhi, Mikuru, and others. Nothing you haven't seen before here, if you've ever read a high school romance manga.
There is one exception: the plot-arc that gives the manga its name. That plot-arc feels more "Haruhi Suzumiya-ish" than anything else in the manga. It has a more serious, supernatural feel to it, and it doesn't really fit with the rest of the manga. But then it's over, and the manga goes back to being about shy people in a zany club.
One of the other reviewers described them as "watered down versions" of the Haruhi characters, and that is basically correct. But again, if you forget that this is a Haruhi spin-off and think about it as its own rom-com, the characters work pretty well.
Sub-par. It fits the silliness, and it gets the job done in describing the story, but it is never pretty, and sometimes weird looking.
If you saw the Haruhi anime/movie and want more of that universe, then you'll be sorely disappointed.
But, if you allow yourself to think of this as just another high school rom-com, you'll find a nice, sweet story about nice, sweet characters, many of whom get their happy endings.