Enemy Within is pretty good. I don't think it should have been sold separately though, it's basically what XCOM EU should have been since the beginning.
That said, I've clocked over 70 hours by now. Really fun, good stuff. As I said before, I like the EXALT missions and the cyber/genetic modifications. This time around I got plasma weapons almost too early in the game though. I feel like tech should be more involved with missions and mission types and approaches should reflect more on knowledge and resources. Otherwise, you might end up advancing faster than the aliens, which is hilarious.
M4xw0lf: if you like the general concept behind XCOM EU/EW, you should try Expeditions: Conquistador. It's a crowdfunded strategy/cRPG hybrid that kicks XCOM in the balls when it comes to actual turn-based combat (and narration).