X-Plane may have the most realistic flight model on the market, but what does that mean if you can't fly the planes that you want to fly?
X-Plane can be extended in nearly any way you can imagine. You can add aircraft (such as those downloadable from X-Plane.org) or custom scenery (again, check out X-Plane.org), or you can download plug-ins (XSquawkBox, used to connect to the virtual air traffic control network, is a popular one) that can radically alter the functionality of the simulator. For instructions on installing these, see Chapter 7 of the X-Plane manual.
If you don't find the aircraft, scenery, or plug-ins you're looking for, you can create you're own. A good place to start is the House of X-Plane, managed by X-Plane developer Ben Supnik. There, you'll find links to the plug-in and scenery development kits. Of course, the Plane-Maker software (found in your X-Plane 9 installation directory) can be used to create aircraft--for more information, see the Plane-Maker supplement to the X-Plane manual.
Using the USB keys availabe at the X-Plane store, the simulator can also be upgraded for commercial use, as well as for FAA certification. These keys will also unlock EFIS-App, used to accurately simulate the electronic flight instrument system found in Cirrus and Eclipse jets.
Prepare to be stunned by X-Plane. Videos and photos speak where words can't. See the Multimedia page...
Get more opinions before you buy. Read reviews and connect with current users. We're confident they feel as strongly about X-Plane as we do. Learn more...
Tens of thousands of users have done it already. Switch to X-Plane and find out how accurate a computer flight model can be. Don't worry... You can bring over your old MSFS scenery, and you'll find that creating aircraft in X-Plane is much easier than in "the other sim." Learn more...
It's blade element theory, of course. By breaking down every surface of the airplane and calculating the forces acting on each piece individually, X-Plane delivers the most realistic flight experience available. Learn more...
The demo is free, and it's a relatively small download. The updates, too, are free, and their bug fixes and new features make them worth the time it takes to get them. See the Downloads mage...
In either PDF or Wiki form, the X-Plane manual can answer many of the questions you may have. See the Support page...
XSquawkBox.net: The X-Plane Scenery Development page
Scenery Central: The X-Plane 8 scenery page
The X-Plane SDK: "Plug-In Headquarters"
X-Plane Airport and Navigation Data: The source for data updates and designer information.