Well I can honestly say that I am neither surprised nor ever doubted anybody other than Snape was the Half-Blood Prince. While this fact has already been established, there are some that still continue to confuse me... or at least make me think.
First of all, JKR titled a whole book after the HBP, and yet I feel like I know virtually nothing about him. To me it seems silly to have a whole book titled after a character the readers do not know too much about. So I'm thinking that there has to be more to the story. Remember, after all, that HBP was going to be the title of Book 2... so if JKR was in any way planning of disposing such a fact to us so early in the series, well there must be some deeper meaning to Snape being the HBP. Something we have yet to have learned.
I also think that through the Advanced Potion Making book, we are able to see a different Snape. We are able to see his brilliant mind and sharp wit. I find it refreshing to learn about him in such a way. I think this also shows how the relationship between Harry and Snape could have differed if it were not for the grudge and strong hatred between Snape and James/Sirius. I think Snape could have been a strong role model for Harry under different circumstances.
I guess all I am trying to get at is that I suspect there is much more to the HBP storyline than what we are told...