I cannot say much about the plot of this book without spoilers, so I won't say much about the plot.
I can say, however, that it is as fast-moving and as spell-binding as her other books. I think, in fact, that this one was much better than most, as it moves well with very few side issues.
As to the plot, it does begin to deal with the dating issues, as Harry, Ron, and Hermoine are all now 16. It also reveals some secrets that we may have wondered about, including how Voldemort survives and where Snapes true loyalties are (maybe--Rowling does tend to make things very complicated). I have to admit that I enjoyed this book, even with parts that will make some people very unhappy, more than any since the first Harry Potter. And it's so nice to see the characters growth and not just in height!