Wanting a clear run at reading it, I waited until this weekend to down it in one.
My verdict: the best book on the why of game design out there. Given that its main competitor is the first edition, that's not perhaps entirely surprising, but it's still worth saying.
I don't actually buy the central argument that games are all about learning; for MMOs, I prefer Sutton-Smith's characterisation of Play as Self over Play as Progress. Of course, if what you're learning is who you are, then the former can be couched in terms of the latter; then again, if what you learn is directed by who you are, the latter can be couched in terms of the former. As with the other rhetorics of play, it really depends on what you want to say as to which position you take.
Oh, and just to show I really have read it, I spotted a typo on page 158: the third-from-last word should be "through", not "though".