Well, I don't know if you've read the Song of Ice and Fire novels it's based on, but in my mind they're the best thing to come out of the fantasy genre since The Lord of the Rings, no exaggerating. Now, it's not for everyone. Westeros is dark, gritty, and unpredictable. It's a sexist, prejudiced, and strongly class-divided society. There are very few pure, virtuous heroes, and a lot of those die - there are many good characters, but they all have flaws, and as the story goes on you see redeeming qualities even in the villains. There aren't tons of monsters to slay. Most conflict will be between humans. It's also one of the most detailed settings I've ever read, with huge amounts of backstory (primarily centered in the last fifteen years but also stretching back hundreds and even thousands). I'd wanted to roleplay in this setting for a long time, so I welcomed the creation of the RPG.
The book has tons of detail in it - you would probably be able to run a campaign fine without reading the books (it'd be off on some details but that's ok if you haven't read them). However, it does contain quite a few spoilers for the first novel, and I'd recommend reading that first.
In any case, I didn't think d20 was well-suited for the setting, and I still don't really, but GoO did a much better job of it than I expected. There are nice gritty combat mechanics, classes that aren't necessarily focused on fighting, and a defect system so you can get something back for playing an insane, crippled, ugly noble. So, while I'd have preferred a GURPS sourcebook or something of the sort, d20 works pretty well and is probably better for certain types of campaigns (such as Night's Watchmen making excursions into the frozen North, or a pure military campaign). There are also very nice social status, reputation, and influence mechanics that lend more mechanical credence to noncombat activities and a character's station in life. I know a lot of people who have lifted these mechanics for other games.
When it comes to magic, it's very rare in the setting. Most people will either say it no longer exists (having died out with the dragons) or is the work of vile demons. There are of course supernatural elements and magic becomes more prevalent later in the series. The AGoT book is based mainly on the first novel, however, so there's only a little - a few 'Legendary Feats' with supernatural abilities and a prestige class that can learn spells. The spells are generally ritualized and strange - you won't see any wizards lobbing fireballs. Most power is in the hands of the scheming nobles and the big guys with armor and swords.
Overall I've had a lot of fun playing AGoT d20, and would recommend it if dark and socially complex fantasy appeals to you, or if you'd just like some gritty combat and dynamic noncombat rules to use.
I should note that GoO is pretty much defunct - if you're getting AGoT get a hard copy, it's probably a poor idea to order it online (though some people might be able to recommend somewhere where you can be sure of getting a copy). It's in limbo right now, as GRR Martin tries to extract his license from GoO and give it to another company, but hopefully he'll succeed and we'll get some sourcebooks (the next one they'd promised would have the very important mass combat rules - as the setting is often about big battles - and more magic).
