Apocalypse has some of the former X-COM glory and adds some nice elements of it's own.
Combat: The turn based combat was done @ least as well as in the previous two titles. However they also added a very good realtime option for combat. Initially I scoffed but finally realized that the same tactical strategies worked with realtime as they did with turn based, and often better (while not taking hours).
Graphics: Much higher quality graphics, as you would expect for the number of years between the original and this one. They also adopted a nicely stylized design angle. From the samurai looking armor to the over-the-top 40s - 60s vehicle designs. The alien designs were nice, and the city in general, good.
Enemies: The enemies were generally less campy and very interesting with a logical progression of difficulty and an interesting biotech twist.
Strategy: Expanded in all the right places, more options for vehicles, on the vehicles, more to research, etc...
There were a few aspects of the game that slipped too far away from the original vision.
The single city state was not near as satisfying (or desperate) a feel as the world view.
It felt too localized for the invasion to feel all that threatening.
The environments, while nice, did not maintain the same gritty, eerie ,and/or suspenseful feel.
I very much missed both of the above.
I think the variability would best have been served in the different tactics and abilities of the new alien threat while leaving more of the game mechanics in place that created the environment of the originals.
A very good squad based tactical combat strategy game that started straying a little too far into being a different game for my tastes.