I don't know why, but this ONA managed to give more to the plot than 2nd season itself in three times less time. Though it introduces a lot of new characters, it's easy for one to orient self, especially if you have already read the manga. Overall it's very similar to main series, so don't except anything extraordinary. The characters haven't really changed and there is put more focus on some side characters, but without getting in the way of the main story. I enjoyed to the fullest and recommend it to anyone who liked 2nd season. The only problem is that it's way too short.