It's a whole different sim. FSX has much better ground textures and terrain. Orbiter doesn't have much in the terrain department except meshes for some areas. There is an add on in development that will add landable terrain but it's still in public beta. Orbiter is meant as a space flight simulator so the ground environment is a little lacking but with level 10 textures the view from orbit is amazing. What Orbiter provides is fully realistic physics of space flight. Travel to the Moon and all planets of the solar system and there are even add on solar systems available for the scifi fans.
Installation is easy. Download two .zip archives and unzip them into a folder anywhere on your computer. If you don't like it you can uninstall by simply deleting the folder. Nothing in the registry. You will also need an add on for sound. The sound utility is written by the same Dan Steph (Dan Polli) who created FSPassengers. You can find a link on the main Orbiter site to his Orbiter website. While you are there check out the Delta Glider IV. It's systems are fully functional and complete with a living crew that can die if you don't take on enough oxygen, subject them to high G for too long, reenter badly or crash the ship.
DGIV also has a really cool heat shield effect.
