I love Star Wars: Rebels. It is, I think, a better show than Clone Wars was. That may have affected my enjoyment of this tie in comic, because I was already familiar with the characters, particularly Kanan himself. (Yes, I know he's still going by Caleb for the majority of this book, but I'll always know him as Kanan.) Then again, maybe not. I do feel like you basically have all the information that you really need to know about Kanan at that point in his life. It's definitely true to the characters as seen on the show, which is what I'd expected. It is, after all, written by Weisman himself. In all honesty, if it hadn't been written by him, I probably wouldn't have bothered with this comic at all.
From the cartoon, we know that Kanan had been a Padawan when Order 66 went down, but we don't know that much about what happened to him after. This is Weisman's attempt to fill in that (sizable) gap. And it works, to a certain point. The thing is, this is a particularly rough part of Kanan's life, and he's not really a likable character through much of the book, for reasons that do make a certain amount of sense in context. If you don't already know his character from later in life, this will not endear him to you. Even if you do, it doesn't really make for the most pleasant reading experience. And there's a somewhat convenient side plot about two clones who had served under Kanan's dead Master, which feels kind of shallow. I know Weisman can do better with this idea, but it's like it didn't have enough room to grow.
This is something that I would recommend to fans of Rebels, especially those who like Kanan. If you already don't like him, or if you have no idea who he is, it's an ok book, but definitely not required reading. But if you're a Star Wars fan who isn't watching Rebels, let me strongly suggest that you do so.