Flight Gear is open source. What it doesn't have, can be added. If more people worked on flight gear voluntarily, it could have already passed up the payware sims. I periodically download and try it ever couple of years, especially after major releases. But the available library is a motley crew. Some aircraft are up-to-date examples of what can be done with the current release, most are old with poor visual quality and/or poor systems/flight modeling. Flight Gear is proof of what can be accomplished with open source and all-volunteer labor... a solid sim with tons of potential. But it is also proof of what cannot be done: its present form is far behind in visual quality to its payware competitors like FSX, P3d, and X-Plane. But with some real support from combat flight sim fans, it could quickly evolve to be better than SF2 in terms of the aircraft and systems modeling. But like FSX/P3d with TacPack, it is a long road to becoming a decent combat flight sim with lots of AI controlled objects that interact and fight with realistic sensors, weapons, etc.
Since it is free, everyone who likes flight sims should try Flight Gear. It can be a paradise for modders.