It's about time this game came to Steam!
This game is a worthy successfor to the original x-com, and is one of the high-water marks for the genre. It takes all of the core ideas of the original x-com (many of which were sadly cut from Enemy Unknown) and adds a lot of meat to them. The "geopshere" world map phase of the game is particularly excellent. The aliens have a plan, and it's up to you to not only figure out what it is, but how to stop it.
One of the greatest improvements is that the game is "pause and play." You can pause the game at any time (and the game will automatically pause when certain events occur, like spotting an enemy), and issue orders to your squad. Every action has a timer associated with it to tell you how long it will take to complete, rather than having action points and a "you go, I go" turn-based system. Things make a lot more sense as a result, and you can plan tactics without enemies taking their turn to run across gaps or cross the map without you being able to do anything about it since it's not your turn.
It also lets you have multiple soldiers moving at the same time, and group-select and move orders, which means that the squad sizes and maps can be *far* larger without the game slowing to a crawl as you move an entire team one at a time down a hallway or street.
The one thing I really miss that was in the original xcom which is missing from UFO: Aftermath is the environmental destruction and the interesting tactical options it provided. Also, this game was significantly superior to its own sequels.