I wouldn't describe it as the worst but I'd say it is not gripping. Half Blood Prince and the Deathly Hallows are gripping. Remember, the 5th book was also a very dense book because JK Rowling wanted to fill in a lot of information that hadn't been given in the preceding books.
So, in terms of how it keeps you excited, it failed but the book packs a lot of other useful information which were referenced in the books that follow.
It could have been, but it was also the cultural event of the year... and it was a book. I grew up in an era when "bestsellers" were written by the likes of Harold Robbins, so to see a genuinely massive popular attachment to books of the (variable but always good) quality of the HP series was a wonderful thing.
Well, I've never read the 6th and 7th, precisely because I lost interest after reading the 5th, which I found confusing and unengaging. That said, I was relatively older when I read the fifth book, so my tastes may have changed in between.
Personally, I found all of them to be similar quality-wise (though obviously varying in the tone in which they were written, as they catered to a more and more mature audience) until the final book, which -- and this is a point of view not many of my friends shared -- I found both appropriate and head and shoulders above the others in terms of both depth and character development.
I remember there was more than a couple of years between the 4th and the 5th book's release. More than enough time for the general audience to grow out a bit of the books.
Personally, I hate the last book but all of the others are just fine.
