FAS : strong boom+zoom fighter, weak shields, strong hull, weak internals. Weapons - 2 large, 2 med.
FDS: multi-role, decent standup fighter, moderate maneuverability, slow speed. Weak shields, strong hull, good internals. Weapons - 4 med, 1 large.
FGS: flying tank. Very slow speed. Weak turn speed. Weak shields. Strongest hull in the game. Weak internals. Weapons - 1 large, 4 med, 2 small.
The FAS is getting a good reputation for being a strong and agile dogfighter. People complain about shields, but most analysis seems to be on paper at this point, people who actually have flown the ship report good survivability.
The FGS is getting a rep for being a bit of a dud. Doesn't have significantly more firepower than the FDS, it's slower, and the internals suck. It's hard to kill though.
The FDS seems likely to rise from the ashes due to an increased relevance of hull in 1.4, and the powerplant sniping nerf, making it significantly more survivable.