im not sure how much i enjoy it if im fully honest. i was sat there the other night doing the same mission over and over cos i was stuck after doing a "storm a building, shoot the guys, chase one that gets away" type mission for the 18th time that night and i thought , as stunning and massive and brilliant the game is, i think the missions are annoying if anything. and if your really stuck on one you should just not have to do it. its a bit too rigid.
Yeh missions are all very samey. I didn't really find any of them too hard although I did fail 43 missions
If its a storm the building mission then it's best to just get a taxi to the destination as it saves a lot of time and will probably cut down that boredom from having to repeat it if you keep dying.