The Story of Kullervo, as you may or may not know, is a short text that Tolkien wrote around his early twenties; it's a reworking of a section from the Kalevala, that infamous Finnish text that I hear so much about and yet can't recall ever reading anything from myself. In itself, it's kind of interesting to read. But what is more interesting and what makes it worth reading is the connection to Tolkien's later works, namely of course the story of Turin.
There was a phrase that Verlyn Flieger (who edited this text and also provided commentary) used: "it is a significant step on the winding road from imitation to invention" ("Tolkien, Kalevala, & "The Story of Kullervo" 163). As a writer and a student of literature, that's what was most interesting. When you are young, you do draw more from what's around you as you're realizing what you like and what calls to you and trying to develop the way in which you'll use those influences while also adding in what is unique to you--it takes time to develop from inspiration into creation. When reading this text, a story that is not from Tolkien, it's easy to see what about it compelled Tolkien--because those are the things that he would later work into his own writings.
The actual text is quite short, only about thirty-five pages. I do recommend reading Flieger's Introduction; she does a good job of concisely explaining what the Kalevala is, how Tolkien came to be familiar with it, and whatever else the reader needs to know before going in. I didn't read her notes and commentary for now. There are also two essays by Tolkien; I skimmed those, as well as Flieger's accompanying essay at the back of the book. If I ever want to go back and read in detail, it's all still there--but for now this was enough.
That's about all that I have to say. I appreciate the publication of this text, I enjoyed reading it, and it makes for an interesting study of Tolkien's development as a writer and a creator.