In one word: absurd.
X-COM, the first part.
It's obvious that XCOM was outstanding, one of the games that deserve to be called classics; a game hardly to be beaten, by sequel or not. But TFTD is simply absurd: it's the same game, exactly the same game. Even worse, the plot is also absurd. What has done mankind with all those plasma cannons taken from the aliens in the first episode? Why must I walk the path again from... harpoons? TFTD is the result of following a success and taking it to the worst extreme: to make an exact copy of the previous.
The Bottom Line
If you have seen XCOM, you have seen TFTD. If you don't, skip the latter completely and try the first.