Honestly I was pretty reluctant to read this because I'm so damn tired of the "mysterious portals full of man eating monsters" plot, and the end result is usually something dumb like "invasion for the sake of war", but I'm really enjoying the little twist the author put on it.
And I usually rate stuff just below their current rating, but this is still underrated. The growth of the characters and plot are amazing, and I can't think of any faults even though I've read up to chapter 86. It at least deserves 9-10 stars.