Personally I love Annette's voice. I think it fits the band's music a lot better.
I picked up Dark Passion Play at Best Buy a couple weeks ago. First Nightwish album I've seen released in America retail stores.
Don't get me wrong, I loved Tarja's voice, but sometimes the operatic style felt too out of place for me. Plus you can't understand her half the time. Remember the Fishmaster? (Youtube: Nightwish Misheard lyrics).
Annette's style is refreshing to me, easier to understand, and fits the band much more. As for the band itself, their style hasn't changed, so it's not like getting a new singer changed how they sounded.
That's just me though. I know a lot of Nightwish faithfuls were pissed that Tarja got fired.