I just finished reading H2 the manga. I have to say that I was confused at the abrupt ending. But after reading Tensai_sakuragi's comment (he's really a tensai), it cleared things up so much (not all, but it helps a lot) and once it does, that when I realised why this series is so good. There is so much subtleties that adds up together at the end, that's what make Adachi such a great mangaka. He definitely fooled me into thinking who should win.
Amazing ending. Author left hints to deduce what happened, but open enough to let our imagination complete it. It was a very very satisfying closure. Like when you just finished putting together a puzzle of your favorite painting.
Although I wish I could see Hiro hold Koga's hand one more time. I'll have to do it in with my imagination. Maybe that's why it's so good, cos I get to envision the way that Hiro express his love to Koga with his little actions and luckily, Koga gets them.
The ending was so deep, and that is why some people don't like it. It is not obvious and it takes a lot of reflection to comprehend. in fact, I wonder if the anime's ending is the same? I don't think the kids are going to get it. They certainly did a poor job in the TV series that I watched some years ago, and that's why I didn't read the manga until recently. Don't waste time on the TV series.
in sum, it was addictive, well developped, thrilling, funny and complete.