Firstly you really don't need to buy the disks; the default installation is available for free from here http://home.flightgear.org/download/main-program/
The download is 1.5gb; and the default location is PHNL Hawaii; lovely scenery and active volcanoes in the area.
FlightGear can download scenery automatically as you fly - there can sometimes be a delay but it usually arrives.
This comes with the default C-172; but there are lots of other aircraft available from the default hangar - all of which can be downloaded easily. However be aware that because of the opensource nature relying on contributions the aircraft quality is variable.
These are definitely good aircraft that I'm aware of; this is just my list and I might have forgotten something great.
777 707 Tu-154 F-15 (windtunnel aero) F-14 (windtunnel aero) F-16 (windtunnel aero) Mig-15 Mirage-2000 Space Shuttle (windtunnel aero, most accurately modeled version for any sim) Robin DR-400 CH-53E JA-37 Viggen (windtunnel aero) Citation II DHC-6-300 Twin otter Alouette-III Beechcraft 1990 Extra 500 A-10 A-6 EC-135 and the EC-130 IDG A320 (github) IDG MD-11 (github) Bell 412 (FGUK.me)
There are some really great models you can fly; I'm going to recommend the F-14 and F-15 (because they're mine), but the Space Shuttle is really worth a look.
The IDG A320 is also excellent; but still in development and only available from github.
There are aircraft available for FlightGear outside of the default hangar; some excellent helicopters can be found on FGuk.me
Overall the weakest and most variable part of FlightGear is now the scenery; so if you pick your location well it's a great overall experience. The grand canyon is good, LSGS, LOWI, LFPG, Jan Mayen are all worth a look. Generally the scenery in europe tends to be better because it derives from Corine which is higher resolution.
http://wiki.flightgear.org/images/thumb/0/00/CH53-E-approaching-USS-Vinson-battle-group.jpg/800px-CH53-E-approaching-USS-Vinson-battle-group.jpg