Even those of us who knew from the start that Snape would turn out to be one of the good guys really learned some interesting backstory in that chapter - she convincingly explained his motivation throughout all the previous books. I also loved the way she parallels Harry's experiences at Hogwart's with the stuff his Mom, Dad, and Snape went through. Snape was in a sense as much a father to Harry as Dumbledore was, even though he chose to hide that affection from him. And it was appropriate that Harry's revelation that Snape cared about him came when it did - at the pivotal moment before Harry fulfills his destiny. Snape's memories serve as the catalyst for Harry's victory, not just by giving him clues and information, but also bringing him into adulthood emotionally. When he accepts and understands Snape's role in his life, he is able to summon the strength to face that final challenge. The way Rowling built the complex relationships of those 3 characters - Albus, Severus, and Harry - was really clever, and she managed to make it pay off in quite a rewarding way, I thought.