I adored Decagon House. Yes, it takes some leeways here and there, but not enough to be troubling. Decagon House is an interesting twist on one of my favorite books (and old mystery movies), Ten Little Indians, written by the one of the greatest mystery writers of all time, Agatha Christie. There's also a nice homage to Sherlock Holmes with some Holmes & Watson moments. Seven college friends and lovers of mysteries arrive on a supposedly deserted island only to find themselves being picked off one by one. Who is the culprit? Will they figure it out before the last of them is killed? The idea and thought of it is nerve wracking.
The story is solid. There are no extra characters meaning there isn't anyone in the story that doesn't give you some kind of information to help you make a decision. Because it is a manga there are clues in the panels that the reader may not pay attention to. I dare say, 99% of the readers ignored it. I had the question in my head and moved right passed it just as the writer probably intended. Personally I think the story is woven extremely well, especially given the length. Almost everything you need to figure it out completely is right there. Actually, I almost had it. I had the murderer from the start, but not the why. That little tidbit remained a mystery until the end. Sherlock's usual. Basically, the writer took a Christie mystery and had Sherlock Holmes solve it. Hercule Poirot was busy.
Reading it took no time. It was like binging an anime or reading a book you can't put down. If you're a mystery fan, especially of the old guard mysteries of the early to mid 1900s and/or the subsequent b&w movies that were made based on them, I think you'll enjoy Decagon House.