My favorite part of this game is the ship designer. You have the stock ships, which are good for learning the ropes, but then you can design your own. You can design sleek, fast, quick building but weak fighters, huge super battleships, missile frigates, energy tanker support ships to keep your energy hungry weapons on that super battleship firing, just about any combo you can think of.
Then you can play your designs on player created maps with unique challenges. Basically endless replay ability.