Overall: There may not be a better book on a given setting out there in my opinion - period. Yes, to me its that good. Also as noted above, GOO is now out of business which is a huge bummer. But in regards to AGOT, this really isn't that big of a problem as everything (or 98%) you need (and more) is in that one book. Sure there might be a minor thing here and there in the later books that would be nice to see in official print but given the tools that this book provides, you can easily create/convert it yourself.
Deluxe Ed: Depends upon yourself and what you like. If you are only going to run a d20 campaign then it might not be worth it as the only difference in the books is a writeup from GRR Martin himself and the complete TriStat rules in the back (only in the Deluxe game). But, even if I never actually use the TriStat rules, I absolutely prefer my Deluxe copy to my standard copy for the "feel" of it. With the leather cover (and the nice alternate coversheet), this book feels awesome in my hands as I read it. Yes, the leather cover might not be important for the value of the game, but to me, it is totally worth it for the value of the book. I gladly paid $100 for the deluxe edition and there is no question that it is the "prize" book in all of my insane RPG collection. You will not be disappointed.
Todd B.