My take:
Unofficial Guide - Great resource, but often overwhelming and doesn't change a whole lot from year to year. I first purchased back in '97 and again in 2002. Not a whole lot was different (aside from the obvious adding/deleting of resorts, rides and restaurants that have changed since.) If you can get a copy at your local library that's a couple of years old, you won't be missing much over the most recent version.
Birnbaums - Great for the kids (with the color photos and such), but mostly fluff. There are a few tidbits that you won't pick up elsewhere, but being the "official" guide, you mostly get the Disney party line.
Passporter - I didn't buy this until after our last trip, but it's my new favorite. Very concise. Probably has as much useful info as the Unofficial Guide, even though it's about 1/3 of the size. Doubles as a vacation planning tool.
You can't go wrong with either the Unofficial Guide or Passporter. Unless you really want to immerse yourself in vacation planning, I think the Unofficial Guide is just too much for many people. I've never heard of anyone being disappointed with Passporter.