I love the 2nd book! It used to be my favorite, before OOTP and DH came out. Now it's my 3rd favorite.
But I remember after I read it, a lot of people said they didn't like it, and I was surprised because I thought it was great.
Now that the last book is out, I really see the importance of the 2nd one. It's very crucial to the whole series. I still love it.