This review is for the Spain & Inca Civ/Scenario Pack DLC only. Familiarity with the base game is assumed. You already have this DLC if you bought the "Complete Edition." The DLC offers good additional content but the store page could be a lot clearer about what you're getting, hence this explanation.
This DLC adds the following content to the base game:
* The Spanish Civilization
* The Inca Civilization
* The Conquest of the New World Scenario (original version)
It is important to understand that this DLC was made somewhat redundant when the Gods and Kings expansion was released, and if you have G&K, you may not want to buy this separately, because: 1) the Spanish civ is also added in G&K, and 2) there is a modified scenario called "Conquest of the New World Deluxe" which utilizes G&K content, and is available as a free download. In other words, if you already have G&K, what you are getting with this DLC is the Inca civ and the original version of the CotNW scenario. Now you know.
As with every other civ, the Spanish and Inca have special units and abilities. The Spanish, lead by Queen Isabella, have more powerful mid-game units - the tercio and conquistador (replacing the musketman and knight, respectively) - and a unique ability that rewards settling near natural wonders. The Inca, lead by Pachacuti Inca, have a more nimble early archer unit called a slinger, and bonuses related to hills, incuding a better farm and faster movement. The civs themselves are well developed and enjoyable to play.
The Conquest of the New World scenario is really well designed. You play either a European or Native American civ beginning in the year 1492. Unlike the base game, you win by accumlating victory points during a 100-turn challenge. The Europeans (Spain, France, and England) race from Europe towards an undiscovered western continent. The Native Americans (Iroquois, Aztec, and Inca) are already there and vie for dominance against each other and the new arrivals. The "New World" continent is randomly generated during every game, which keeps things fresh and, IMHO, maintains the spirit of exploration and discovery even if it does give up geographic fidelity. As such, the scenario has high replay value.
This DLC adds some great content, but be aware that you might not want to buy it separately when it is available in the Complete Edition and, in large part, in the G&K expansion too.