Great game, but you may want to consider not buying this for a while...
In general, FSX is similar to its previous release FS2004(a.k.a FS9) with a few addition. They have added new camera angle on the airplane (wing view and landing gear view, for instance), and many detailed airports in many countries. I can even see Air Canada sign on a cargo building from Toronto Pearson Airport.
AI's ATC (Air Traffic Control) has also improved, as their instructions are usually reasonable and correct most of the time. Although they might miss to tell you to stop for a small truck that blindly drives right in front of your 747 while you are taxing, you can say they are pretty much reasonable during the flight.
Why would I reconsider recommending this great and improved Flight Simulator X, then? It all comes down to hardware problem.
With hardware that meets recommended specification, you still can't even go close to half of any settings. As a result, if you want to run this game smooth enough to play it without any problem, you have to turn down the graphic so low, it basically looks like FS2004. Even with the addition of Windows Vista and GeForce 8800 video card that would cost round $1000, you still can't run it well enough to see all these stunning visual effects.
Well, if that's the case and if you have FS2004 already (and if you're reading this, you probably would), I don't see any reason why you should buy FSX as FSX is nothing if you exclude that visual effect.
Unless you are a hardcore FS fan that must try out this game, I recommend you not to buy this game just yet, because you may feel cheated for the money you paid for.