I'm about 140pgs into the Agile book atm and I'm working on adding the check-out feature to my shopping cart. I'd have to say at this stage I'm very curious at learning more about the framework. As you've mentioned, this book does seem to start from a more advanced level.
I'm understanding everything so far, but don't know any of the details about the framework. Pgs 228-598, however, are dedicated to covering the framework, so I'm thinking that's when you really get to know what you're doing. Did you read the entire book Haris?
I've programmed PHP for around 5yrs and I would recommend you learn both PHP and RoR