Kadlin: It's a good tactical game where you can level up your squad members, and outfit them with new weapons and armour that you research and find in the field. It's real-time with pause, not turn-based. I don't think it's as good as the older tactical games like X-Com and Jagged Alliance, but if you need a squad based tactical game fix, it's good for that.
You won't find a consensus on which UFO game is better, people will like one over the other. I think the atmosphere is the best in Aftermath, in later games it becomes very cartoony, but the combat is done slightly better in my opnion in later games.
My only gripe with Aftermath is that you have no input over when you get new recruits. I've found that tactical game-play in it to be superior to aftershock with corner leading, non-useless grenades and launchers, and a wide variety of useful armors. In comparison, aftershock feels almost MORE like a RPG than tactical game than it's predecessor. (I haven't gotten to afterlight yet.)