Make the most of it by ensuring that most tiles around you have at least 1 gold and 1 hammer so that you practically double your production and income.
The immortal isn't bad either, but the Archaemenid Legacy is where it's at with Darius. The Satrap's Court is a nice uncompromising upgrade as opposed to a regular bank. Money and happiness go hand in hand I always say. Medic is a great upgrade for them when taking out barbarians.
Darius I Civ Bonus: They also heal at double rate.
