This story deals with Levi and his small band of misfits who dwell in the underground. They cross paths with a younger Erwin who is a squad leader at this point. Levi and Erwin's relationship in this manga is much different from what you would come to expect via Attack on Titan. It is very interesting to see how everything plays out (There is a small subplot that helps drive the main plot, and this contributes to the two being at odds). Levi gets further development and you see a slightly more primordial version of the Survey Corps. My only gripes are that the story is too short and you don't get to really connect with the newly introduced supporting cast.
The art is crisp and similar to Attack on Titan. I perceived a wide range of physical differences between characters, so it's easy to tell people apart. The environment has a genuine feel to it that brings the scenes to life.
My favorite parts of this series are the characters and their interactions with one another. The situation that Levi and his two companions find themselves in really lets each of their qualities (good and bad) shine through. All three of them are quite different and their conversations can be fun and interesting. Erwin also is shown as a great military mind who is on the verge of revolutionizing the Survey Corps. It's cool to see his rise to prevalence.
Overall, I am sad that this series only has two volumes. It sets things in motion and then wraps it all up rather quickly. The story is relatively simple and there really isn't much of a twist to speak of, but the ending does provide a pretty good explanation as to why Levi carries himself the way he does. It also leaves a small question in the back of your mind about what Levi's final destination will be.
This is a good read and any fan of Attack on Titan would likely glean at least a decent satisfaction from what it has to offer.