My wife and I spent a good 30 minutes sitting on the floor in front of the WDW books at Borders. We looked mostly at Birnbaum, Passporter, and UG.
In the end, we couldn't decide, so we purchased both Passporter and Birnbaum. UG was a distant third for us.
I think our Birnbaum purchase was more for tradition than anything. We've purchased one for every one of our trips since the early 80's. I don't really like it now that Birnbaum is dead - seems so watered down, generic, and unwilling to be very critical of anything (since it's produced by Disney).
For anyone relatively new to Disney, though, it has lots of pictures and isn't so information heavy that it is overwhelming.
The Unofficial Guide indeed looks to be a huge wealth of information about all sorts of things. However, the layout of the book is very bland and uniform, making it hard to flip through the book to find what you are looking for. The structure is not obvious at a glance. The book is also rather hefty (at least from our perspective, compared to Birnbaum). A significant portion of the UG is about traveling to WDW and hotels off-property, which is information we wouldn't use at all. In the end, we could see that UG was NOT going to a book that we could carry around with us in the parks and get a lot of use out of.
The Passporter, in our opinion is the best of the three. The book is DESIGNED, not just written. It is clear that you are meant to carry this around with you during your vacation. There's a spot for a pen, a hard protective cover that folds around the book, an elastic band that can hold your place, and a structure that makes it easy to flip to the section you need to find the information you are looking for. In addition, the book has some stickers and tabs that you can use to customize your Passporter, fold-out maps, and pockets at the back for keeping a log of your itinerary and storing loose items like receipts and times guides.
To be honest, we weren't sure about the pockets and whether we'd use them (especially because it means there is less room in the book for information). However, there's a better chance that we'll use them that the pages of non-relevant info in the UG.
There are lots of other little touches like highlighting textual changes since last year, providing a website of udpates that allows you to make edits to your book (most recent updates were posted about three days ago), and including lots of little maps of resorts, rooms, etc. The dining and attraction listings have reviews and lots of other related info in a handy charted layout that presents a lot of information for quick glance.
Everyone's different, though, and as you can tell, many people swear by their UGs, even though we ruled it out easily. My best suggestion would be to head to your local bookstore and spend some time comparing the different choices.
Happy planning!