I think I've always had a kink for the "reformed bad-boy with a dark past" archetype.

EVERY girl chases the bad boys. >_< Don't even get me started.

If it turns out he was really evil all along, well, that's just disappointing.

[...]'Cause if Snape really is on the bad guy's side, why leave that little bit of ambiguity for the readers? Harry's convinced that Snape is a bad guy. All the characters appear to be convinced. There's no point in letting the characters all be convinced and yet leaving the readers hanging unless the conclusion is ultimately going to be "Wow, he's good after all!" It's too late for the big, "Wow, he's evil!" reveal; Dumbledore's death scene was it.

I disagree entirely. The whole point of the "tricking the audience" and leaving the possibilities open to do either is precisely what she has done. Ignite the fandom, get them interested, make them care. Admittedly, I strongly suspect it's going to end up the same way you imagine, but that's because it's become a bit of a cliché in some ways.