Too frustratingly hard.
I know difficulty is a lame thing to review about, but you hadn't got the enemy difficulty set right. It takes away from the fun when you have to repeat a level 20 times from the beggining to finish, just because some lucky arse headshots you with a plasma rifle.
Then that level where you play as the "friend", what is that all about? You spend all this time building up weapons and armor just to play a level without any, thats where I gave up. Fun up to there, good concept, but work on your damage/accuracy, weapon strength and jump physics. Wouldnt hurt if you threw in one of those jetpacks at some stage either.