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If you played the predecessor then two things really stand out about this one, first the removal of the ability to shoot from corners of buildings.
Second a lot better mission design. You can tell that the developers got more experience in this game and they've put it to good use. But it's still with a lot of flaws and imbalances which mostly relates to the weapons and the ai which is quite poor.
Often you'll find yourself in a mission where the aliens try to run you down like a pack of angry bulls, and you just need the widest spread possible on your guns or use grenades.
Luckily most of these issues are fixed in the excellent Weapon Rebalance Mod by ShadoWarrior which can be downloaded here
A more recent mod and with some scifi like effects is the ACM Total Conversion by Okim which can be downloaded here
Using either of these mods will really improve the playability and enjoyment of the game, it actually gets so good that I've played through it a couple of times, Aftermath I've only been able to complete once in comparison.