I bought XCOM: UD the day it came out in 1993, I was hooked to say the least. =)
Today, I fully support both titles and they both fill a need nicely. I XCOM: EU is a very well done game that reboots the franchise in a big way. It has sold very well and will likely spawn DLC and sequels. It has enough tactical depth to keep me interested for a good while and is certainly a decent challenge to boot. Could it have been better? Absolutely. I really love the ant farm concept if not the actual execution.
People have griped about the interface, I don't know why. I find the interface intuitive but then I find Windows 8 intuitive where tons of people don't. I guess I'm just more flexible than others.
I will buy both games, I will love both games as if they're the last we'll see of the franchise for a good long while again.