Okay, before I totally gush all over the place here, let me point out a few things that struck me as being just incredibly amazing.
First off, the pacing of it was elegant. You have the perfect build-up, and then this lovely climax when Snape has his breakdown, and then a major climax when Harry saves Snape.
Snape buying him the dragon as a way of showing just how far Snape had fallen was brilliant. And then there was the fact that older snape with his memories was the same bastard as always...
It was just awesome. I do somewhat agree with the above comment about the ending being a little abrupt, but I think it's alright. I mean, after thinking about it, the reason I wanted the ending to be drawn out was 'cause I wanted more story - not so much that it needed more at the end.
You've really created a wonderful piece of fiction here, and it's one that I'll come back to read again and again, I'm sure.