Okay then. First of all, this manga is great. Maintaining a consistent level of quality across twelve volumes is an admirable feat for a mangaka. Seven years! I would readily recommend this series to the right sort of reader, with the warning that the ending may be unsatisfying. If I'm to judge it while disregarding the ending, I'd give it a high score.
So, the ending. It honestly doesn't "feel" like it has one. It's almost as if it were actually a thirteen-volume set, and the guy you bought it from, used, at a discount, had lost the final volume two summers prior. Open endings work well for some manga, but not in this case. I think Uguisu-sensei should have just put in a standard-issue "hero saves the world" ending. It would have fit perfectly. After all, as she noted, she put in fan-pleasing stuff such as pretty girls around the MC at the urging of her editor(s). As a story it is, in equal parts, clichéd and inspired, with the clichéd half naturally demanding a neat little bow tie to finish things up.
Maybe it just got the axe prematurely.