I was totally drawn into this story. Ron's all-elbows blushing and completely earnest boyishness, Sirius's predatory designs obscured by Sirius's misguided sense that he himself is just one of the boys -- such a dangerous combination. I thought the way Sirius was going through his family's home and possessions, accompanied both by his gradual discovery of memories *and* his not-exactly-patient seduction of Ron, worked incredibly well.
I really liked that Snape seemed to be on to Sirius, partly (I think?) because of what Regulus must have told him, partly because of his own resentment of Sirius for a variety of remembered wrongs, but *also*, in however small a part, on behalf of protecting a child that he might not much like, but who is nonetheless his student.
Interesting that Regulus was the one initiating -- it seemed possible to consider his overtures as doubly fueled, both by his estrangement with/desire for closeness with his brother, and by the sense that seduction is what it takes to get someone to join the Death Eaters.