I always thought that Snape was a dark person inside and out. I thought he was cruel and unfair and I wasn't surprised when I found out he was (at that time I would've used the word "is") a Death Eater. After I read "The Prince's Tale", my opinion changed completely. I viewed him as a misunderstood boy growing up. I was surprised to find that he was bullied instead of being a bully. He ended up on the wrong path, and once he lost the love of his life, he lost everything real in his life. From that point on, he tried to make things right by working for Dumbledore and trying to protect Harry (although he disliked him because of his father).
I adored Severus before The Prince's Tale. He was my favorite character.
I never thought he was the bad guy, that there was more to him than being the 'greasy git of a teacher.' I respected him and loved him. When I read The Prince's Tale I understood him, felt sorry for him, and loved him even more.
I was also delighted that I could now tell everyone who had told me I was crazy for thinking he wasn't a bad guy and *SPOILER!!* loved Lily, "I told you so!" exuberantly.
To be honest, I absolutely hated him. I thought he was cruel and at times unfair. I especially hated him after he killed Dumbledore. After seeing Deathly Hallows part 2 for a second time, I finally realised he was doing everything out of his love for Lily. (Didn't pay much attention during the book or the movie the first time)