I love games that you can play with one hand. It makes it easier to multitask. Eating an orange, updating Facebook status or changing diapers on the kid at the same time as playing XCOM. This is true for most turn based strategy games, as you can leave and come back at any time.
XCOM: Enemy Unknown is like any other strategy game from Firaxis. It sucks you in hard. "Just one more move" and an hour later you still think, "just one more move". It is highly addictive to see your troops evolve, research new technologies and using improved weapons in the field. It is just as brilliant as expected.
But once passing the twenty hour mark something bad happens. It stops getting interesting. That abduction mission looks pretty much like the previous abduction mission. That UFO looks pretty much like the previous UFO, and you have learned pretty much all the strategies needed to down any foe. Environments aren't giving you much variation anymore.
This can often be repaired by an interesting story that will urge you to continue on your strife and beat the game. Sadly, the XCOM story is just as generic as any alien invasion story can become. It offers no plot twists, no interesting angles and no surprises along the way. It looks more like an afterthought and is there just as a servant to the excellent gameplay.
This is one of the best game I've played 2012, but it is not perfect. I will probably come back to it and replay it on higher difficulty level, but once finishing the first play through I'm not that inclined to do so at once.