I'm going to say this straight up, if you haven't played the Touhou games, it is recommended to not read this manga, as it will not be nearly as enjoyable and is not aimed at you.

The story is, in a brief description, all about a race to the moon. Assuming you are a fan of the series, this is quite the enjoyable read. It expands on the characters and their personalities in ways that the games have not, and introduces some interesting new characters as well. There's some nice action in it that I'm sure all fans of the series will love, as well as a plot that'll keep you wondering just what's going on inside the character's heads. The ending is very ZUNesque, which is to be expected, and the art is very pretty to the eye as well.

I would definitely say this is a must read if you're searching for more from Touhou, and a great place to start with the manga series. The plot is driven through the characters, so you had best be fond of them, or this manga isn't for you.