as many people have already said, the alien campaign is unnecessarily difficult. the levels are made difficult in unfair ways such as outnumbering you with enemies and pairing you with inferior AI allies. now i enjoy a good challenge, but the levels are harder for no good reason and thats where it stops being a fun challenge and starts becoming a mundane task
the game has an addicting appeal to it, but its a little too drawn-out with no purpose to keep playing after your done with the story. also i hate how "quickmarching" actually makes your units move backwards if you're all the way to the right of the screen.
Glad to see that game developers are more concerned with making money than with making games that can be equally fun to all of its players as supposed to fun only for those that have bottomless pockets to waste money with.