The scale of this game is what really gets you. Battles can encompass thousands of units, artillery can shoot further than most tanks could travel in 5 minutes, but the largest maps are so huge that the only sensible way to cross them is with aircraft.
The default unit set is vast - each side should have at least 10 mechs, 10 tanks, 10 aircraft and 10 ships - and adding extra units is as simple as downloading the unit file and placing it in the game directory.
Definitely worthwhile for any RTS fan.