Firstly, let me assert the fact that I am not an obsessed Twilight fan, so I hope my opinion may help those of you who are just 'casual' readers of Twilight.
I, unlike hundreds of others, didn't wait impatiently for this to be released. I didn't even know there was going to be a guide. However, I had received a WH Smith voucher; when I saw it in the shop, I was intrigued. I didn't expect much of it.
However, this book is well worth the purchase, in my opinion. The back stories were fascinating, and I learned so many new things about the world of Twilight; it really adds depth to the reading experience for me. As many die-hard fans have said, some of the information is already available on line (particularly the interview) so if you already know everything about Twilight, there won't be that much new except for the stories (Alice's, Allistair's and Victoria's were my favourite).
All in all, if you know pretty much everything about twilight and are expecting a guide with a wealth of new information that has never been revealed before, you will be disappointed. However, if, like me, you've only read the books a couple of times and seen the films, then this is a very valuable addition to your collection.
Hope this helps!