FSX runs fine on Win7, as long as your hardware can support it. The only two issues I've had:
- On your "sound" control panel, the Communications tab, change the option to "Do nothing." FSX ATC sounds count as "communications activity," so it will reduce the volume of everything else (engines, wind, etc) whenever ATC speaks. This was extremely frustrating until I figured out what was going on. Of course, if you're only going through one set of speakers, you might consider this a good thing, but I pipe the ATC through my headphones and other sounds through my speakers.
- Enabling add-on scenery. The open dialog doesn't quite work right, but there's a little voodoo you can do in order to get it to work.
Other than that, I've had absolutely no issues with FSX on Win7. One of the advantages of it being a Microsoft product is that unlike many other games (including Orbiter), it actually follows the Windows guidelines on things like saving settings and saved games, so it'll work fine if you install it to your Program Files directory or aren't running as admin.