As always it depends what you want to do.
FlightGear doesn't (yet) have a study level modern passenger airliner. GA, military jets and helicopters are well represented, and the 777 or 707 are both great.
The scenery can be of variable quality, some areas are great (LOWI, grand canyon, PHNL) but others lacking. Also the ground station data hasn't been updated for a few years.
However you can get great results both inside and outside the cockpit.
http://www.fguk.me/media/kunena/attachments/667/FlightGear2019-03-05_19-02-22.jpg
http://wiki.flightgear.org/images/thumb/c/c0/SOTM-June17.png/800px-SOTM-June17.png
http://www.science-and-fiction.org/FG/pics/fg_shuttle02.jpg
