An addictive, absolutely enjoyable turn-based/base-management strategy game. Also you get to kill bushels of aliens.
Well, add XCOM: Enemy Unknown to the list of challenging and fun games. I played (and beat) it on Classic Ironman, and I could barely step away from the computer. It definitely has that sense of "just one more mission." 30 minutes before work? Time enough to fill some aliens full of holes!
What I found especially enjoyable were the progression elements; it's fun to unlock new armor and weapons (a jetpack? cloaking armor? a guidable rocket launcher? GIMME). And it's fun to upgrade your soldiers, especially considering how fragile they are. Have them die in a mission and they're gone, forever.
It's that aspect that made the game a real joy to play. Since I played on Ironman (which means I only get a single save-slot, and cannot 'redo' anything!), any mistakes would result in the permanent loss of my best soldiers. It added a whole new level of tension. I couldn't afford to make mistakes. Mistakes could easily compound into a failed mission which can easily compound into a failed game. It's punishing.
But it's fair. It took me roughly 20 games (2-3 which were a great many missions in) to finally win, and every loss was due, at least in part, to a mistake on my part. Generally, I was overeager. I moved forward too quickly. I didn't bring my best soldiers on a Very Difficult mission. I see people complaining about line of sight and shooting that doesn't make sense, and that's simply not true. In my final victorious playthrough on Classic Ironman, I lost no countries and only lost 2 high-ranked soldiers. You just need to learn the rules of the game, and play with perfection. Can't handle it? Then drop your difficulty.
But I would suggest that every person play on Ironman, either Normal or Classic, depending on your level of patience and masochism. Constantly rewinding your game by loading a previous save will knock all the tension right out of the game.
And it's a great game. It may not be as complex as the original, but it's just as compelling and addictive. If you're a fan of strategy games, you should not miss XCOM: Enemy Unknown. (And remember to play on Ironman!)