As the title says, it's like X-Com. It's basically an updated, 3D, realistic, FREE version of the old X-Com games.
http://ufoai.org/wiki/AboutThe gameplay and graphics are surprisingly good for a FTP game. The Earth looks great, and the graphics for battles resembles 2003/2004 graphics, maybe farther behind.
Pros as far as I can tell: Multiplayer, Actual plot for an FTP, Graphics, UI, available for a lot of OS's.
Cons: Buggy as forget sometimes. Also, multiplayer throws you straight into an advanced skirmish with no idea what you're doing most times, some micromanagement sometimes.
I'd say the difficulty is reminiscent of Dwarf Fortress. Extremely hard to get used to without tutorials. And even then...
Youtube tutorial for current version(2.4):
http://youtube.com/watch?v=kfYoSuTUbZI