Hands down, the only choice in realistic flight simulation.
Having flown real single-engine props, I can safely say that the tagline "as real as it gets" is almost true, although the single biggest contributor to the enjoyment of this title is a good flight-capable joystick, preferably with some kind of throttle, a view-direction hat switch, and some extra triggers for flaps, landing gear, and so on. Even more so than a powerful rig, FSX is pretty much useless without it.
As for hardware, FSX is obviously demanding, but with the Service Pack 1 downloadable update and judicious tweaking of the detail settings, it's entirely possible to get performance as good as, if not better than the previous version, which has far less realism in the textures and visuals.
My only gaming rig at the moment is, surprisingly, a Dell XPS m1330 ultralight notebook, and to see FSX run so smoothly and realistically on this sort of gear is nothing short of amazing. The slower prop craft, in particular, feel highly realistic, even when playing on a puny 13" laptop screen.
If you're into civilian flight sims, this is a must-have title.