I've never actually driven around any of the tracks, so my opinion may not mean much to you guys... but from what I've seen on TV, Forza nailed the tracks.
I mean, I ABSOLUTELY HATE racing at Silverstone, but when I watched a race on TV, I was amazed at how everything there looked exactly like the game. Even the dreary gray sky. And I got to see a couple of Road Atlanta races, and again, every detail seemed spot on.
I've been diving hardcore into racers since GT4 came out. I've bought and beaten GT4 and then Toca 2, and I'm working on Rallisport 2, Colin McRae 2005, and Forza. So call me a fanboy if you like...
My take on the Forza vs GT4 thing... I personally think Forza is the superior game by far. I lot of people have their own likes-dislikes about the 2, but for me, things like AI and damage FAR outweigh the number of cars thing that GT4 has going for it.
Forza had a bit of a learning curve, but once I got through that, I find that my driving ability on other racing games has improved. The game actually taught me how to drive better in videogames! And it may start out easy, as Blazin mentioned, but I guarantee you that you will be forced to earn your victories later in the game. Blazin (as well as a LOT of other gamers) mentioned that the Corvette stuff is rediculously hard... well I can smoke that series, but I get owned during the GT class championships. This level of challenge is important to me.
With GT4, I never have any competition. It's like "get in the front... do laps, let the drone cars do their thing" With Forza, it's a tight battle all the way.
No, Forza isn't perfect.
-- I'm a little dissappointed by some of the sound effects. Not really the engine sounds, because I don't know how they "should" sound, but more like how you can hear a car hit a wall 20 yards behind you louder than you can hear your own crash.
-- The AI does get a little too agressive later in the game.
-- The soundtrack is lame (thank God for customs).
-- You can't damage your car to the point where you can't finish.
-- Lame race qualifying system.
-- And the lack of a cockpit/hood view is simply lazy on the developers part (same for GT4).
Although it's not perfect, I do believe it is the superior racer on the market right now. Not my favorite, mind you (that title belongs to Toca 2) but still the best overall sim racer out.
By the way, I've logged about 60-70 hours into this game trying to get through the career mode, but I'm so burned out (as I should be of any game after that many hours) I let a friend borrow it for a while. Also, I don't have Live, so even though everyone says that the online stuff is the best part of this game, I can't comment on that as I've never played it online.