Let's see, what do you need to know about this one? I guess we'll start with, is it entertaining/enlightening? Yes, to an extent, about as much as you'd likely expect. Is it an "essential read"? Probably not. Why do I answer as I did? This book has one new Sookie Stackhouse novella in it, focusing on Sookie and Sam's long-awaited trip to his brother's wedding, where trouble awaits (of course). It offers as much entertainment and fun as you'd expect from a novella-sized mini dose of Sookie. The rest of the book is somewhat enlightening, but not anything I'd consider "critical," even for die-hard fans. There are mini-interviews with Alan Ball about the "True Blood" TV show, and with Charlaine Harris about the Sookie Stackhouse series. There's also a complete series timeline/summary section (complete through "Dead Reckoning"), which basically just repeats what is in the books but does give a solid actual date for all the events, and an alphabetical index of all the characters and entities in the series, which could come in handy if you forget who someone is or something. There's also series trivia, a recipe section, and some short essays about the Charlaine Harris fan club, Sookie's experiences, all of Charlaine Harris's writings, and so on (including a definitive guide to where the short stories fit in the reading order/timeline). There's also a map of Bon Temps, and a Stackhouse family tree. They're a little fun, but not really necessary.
Really, that's about it. So, unless you're dying for every single variation of every scrap of info you can get, you'd likely only truly be strongly interested in the novella. If the book is worth it to you just for that, then go for it. The rest of the content is nice, but not anything to weigh a reading/purchasing decision on, unless you feel a strong need for an alphabetized character index or whatever.
I do hope nobody misunderstands me though. I think this companion book is quite well done for what it is, and they did a good job putting it together and giving fans the info they may want or need, with some extras, just as any good series companion book should do. However, like all such books, the need the majority of fans will feel for it is probably somewhat low. It's the kind of thing most will likely borrow from somewhere or someone to flip through once, but won't necessarily add to their keeper shelves as readily as they might with the main series books.
I hope this made sense, and helped a bit.