Enemy Unknown's standalone expansion is a really worthy one. The story is the same but the additions are many an' good. A new substance called "Meld" is found and is used in two new labs to bio-enhance your troops or make them more or less cyborgs, criticaly adds to the tactics, gameplay and fun factor.
Also add about 47 new single and multi maps, new sound options like the soldiers native languages and some new units and you have all an expansion needs. And last, the general quality of life in the game is better than before. Just go for it.
Story 5/10 Gameplay 9/10 Technical 8/10 Overall 8/10