What a satisfying read! I think it says so much about the development and narrative depth of this fic that, though I had to read it in fits and starts due to RL issues pulling me away, the plot and the characters stayed with me quite strongly, and I had no need to refresh myself on what had happened just previously, because the threads were all very well woven together.
I really liked seeing how Sirius gets his chance at action -- all kinds of action! ho ho! -- here. It's a great contrast to his frustrated isolation and stagnancy in OotP, this AU's scenario allowing him to insinuate himself into the most secret and most dangerous part of the plot to take down Voldemort. It was a relief, as well, to imagine Harry getting more space to have some coming of age without the enormous pressure of the later Horcrux Hunt, though I think Harry might not have comprehended the depth of what Sirius was attempting and accomplishing until after the game had been at last played out.
The hints at H/D were great fun: a nice parallel to the adolescent ire being gradually re-purposed by Sirius and Snape's sexytimes. Seeing glimpses of Harry and Draco perhaps becoming friends, perhaps becoming more, while they are still in their actual adolescence (and that enabled by Sirius's willingness to take on the Horcrux Hunt), provided a sense of how Sirius and Snape could be just as challenged and changed by their relationship though they are of course now adults.
And Snape's sour reactions to Sirius's joie-de-vivre (nice to see that angle of Sirius that must have hearkened back to his young adulthood, though now with the tempering of what he learned and gained through his near-death experience) were great -- I can absolutely believe him feeling trifled with, but also drawn to, this independent and mysterious (and more unflappable) version of Sirius.
Glad they were able to take their interactions to a far more intimate level than either of them could have predicted -- I like that they went through the crisis of revealing Snape's initial reason for sticking around (of course Snape's role in the struggles with Voldemort would have inescapably played a role in his interactions with Sirius), and that they each took great personal risk in order to save one another in the moment of greatest crisis.