Also, embarrassingly, I have never finished Kodomo no Jikan despite how much I love the manga, and despite owning all the volumes (I reread the series each time a volume came out). Got scared away by people saying the ending ruins it >->; but honestly at this point I think I just lost track. I imagine I've been missing out quite a bit.
I'd say read it. I think the main reason why people rail against the ending is because of two things:
1. "man, that escalated fast"
2. "if you squint, you'll see it."
Those are my clues that kind of give away the ending. Basically the story suddenly picked up and then the ending barely ties things together.
Personally, I was happy with the ending. It touched on the important points.