Haikyu!! is definitely one of the only few volleyball mangas/animes out there, and it's a great series.
The main protagonist high school team, Karasuno, is made of quirky and unique characters. It's very much like an "underdog" type of plot. It has a serious side but has plenty of gags or comic reliefs to keep you entertained and the story light-hearted.
When I was reading Haikyu!!, the plot felt very intense when it needed to be, and I was reading each chapter nonstop. That's a good aspect, because I don't think I was ever bored! The plot was moving at a good, steady pace, and the characters always whipped up a new surprise to shock me.
The art is its own unique style, and I enjoy it. None of the major characters look even slightly the same, except for their heights, which is a running "gag" in the series. Since this is originally a sports manga, a key problem in some art is movement. There is a lot of movement, and it's important that a reader sees where the ball is moving and such, I think the mangaka did a great job though. The game movement is quite easy to follow, because sports can get a bit hectic.
You don't have to like volleyball to enjoy this manga. But I have to admit, I'm a fan of the sport now after reading this!
The manga is about 100 chapters (10 volumes) in, after the first chapter, each has about 18 pages, I'd say. I would say the series is still "young." It only began two years ago, and if compared to the other mangas Jump! has, Haikyu!! has a long way to go. Hopefully, the manga can continue with more surprises to come.
If you're more of an anime fan than a manga fan, Haikyu!! has just started its anime adaption. It's only a few episodes in, but the plot and details are almost exactly the same as the ones in the manga. It's definitely a laugh, so check it out!