I'm answering this too, because I was thinking about it a couple days ago.

Rickman's Snape is impossible to hate (for me). He's awesome. Snape in the books is easier to dislike until you really get into the last few. He's still petty and nasty in the first few, and it doesn't ease up much until we start learning why he is that way.

Huge example: in the confrontation with the werewolf, he's an ass through that whole section of the book. Barely redeemable. In the movie, he's a jerk to the adults and not so nice to the kids, but as soon as Rickman realizes Lupin could attack, he shields the kids with his bodies. They give him more opportunities to look human.