It doesn't compare to FlightGear. FG is built with a focus on simulating the physics of flight.. It's seen some really nice improvements in the eye candy department as of late as well.
Plus, with FlightGear, it's free, as in freedom, and as in beer, as in "here's the source code and all the artwork!"..
There isn't a market place to buy new locations because the ENTIRE PLANET is available for free in FG. Not to mention hundreds of planes, from gliders to cargo/passenger jets, WWI bi-planes to WWII fighters and bombers, to F-15's, etc.
Local weather phenomenon, real-time real-world weather conditions from live METAR data on the internet, multiplayer support allowing you to fly with people all over the world, all free.
All for free... Any other flightsim is pretty much DOA as far as I'm concerned.