FlightGear appears to be what X-Plane used to be at version 5. X-Plane (which is now at version 10) is quite impressive and enjoyable indeed. I play it all the time.

Just to note, Flightgear is open source and free. It uses a pretty good flight dynamics engine and the basic aircraft are modeled accurately. The Cessna 172 handles just like a real one. For free, it's great. It's let down, if you can call it that, is the eye-candy is not as good.