A well done rendition of the original XCOM concept from what I've heard, and playing it has been captivating for a first time.
There are bugs but they're not game-breaking at all, the gameplay loop is very solid while the base building part is a bit lacking. The narrative is dated as the original concept wasn't re-treated for a modern game, but it's not the main appeal of the game.
Level are procedurally generated, or maybe it's pre-baked templates with slight changes, some level designs are better than others, but which ones you get is random.
The expansion is more or less playing the game from scratch with extra weapons, mods, and enemies. I expected more from Enemy within so I dropped it.