It is fun. If you fly big realistic airliner addons, some nice scenery addons and a good weather addon, throw in a couple of engine, instrument or hydraulic failures and a realistic traffic addon it suddenly becomes very interesting to the point where your hands start to sweat.
If you are a real pilot as well with access to Jeppesen charts then the realism goes over the roof. I love FSX, it just that it has been neglected for a number of years and the "cracks" start to become bigger with many addons now becoming more and more system demanding, you can't push for more than 30-40 fps even with the best rig on the planet.