Well I've read this cover to cover, typed in every line of code, completed all the challenges and delved into every "For the more curious" sections.
This is remarkable for me as I have 4 other iPhone books and never made it past Chapter 4 or 5. By all means buy all those other books and I'm sure you'll get something from them BUT - read this one first.
Joe and Aaron seem to have this remarkable ability to a) explain things in a manner that you understand and b) anticipate where you're likely to need extra help. Even the challenges at the end of most chapters are perfectly designed to push you on that further step to really confirm your understanding. I am not connected to Big Nerd Ranch in any way but it seems that they have tailored the book to reflect the first hand experiences of developers who attend their classes. Hence the uncanny anticipation of your next question.
To get the most out of this book you need to know the C language - not to any uber advanced level but enough to know about pointers, memory management and the general datatypes and flow of execution. It also helps to have a reasonably sound understanding of object oriented techniques and principles.
The book takes you through all the sexy stuff, Maps, Animation, Graphics, Accelerometer, Camera, Web Services and basic Audio and Video as well as the necessary building blocks of View Controllers, Tableviews, Address Book, Localisation, Bonjour, Settings and all the storage options. There is also a responsive forum at their website that Joe and Aaron personally monitor that helps address any further questions you have and occasionally sets a few extra challenges.
I can't recommend this book enough - sorry if it's coming across as gushing but I've had an idea for an app for some time now, and after reading this book, I feel like it's going to happen.