I really enjoyed playing FarCry. It was difficult for me at 'Medium difficulty'. I still havent got past the very last stage where you have to descend into the volcano, its insane!
The game made me change the way I played to be successful. I stopped doing simple 'run and gun', my mode of operation in the other shooters, and instead started to use much more stealth. I found taking out several enemies without them having a clue where I was to be very rewarding.
I also enjoyed Halo because you could only carry a couple of weapons and had to be very careful choosing what you kept with you, how much ammo you had etc.
For us that grew up playing games I think the difficulty is fair, for people new to PC gaming, I feel for them, having to use a dozen or more keys with one hand, getting used to mouse control!
I do like the option to use codes to help me or unlock certain things, if and only if I am truely stuck. What I really dislike is games like Colin McRae Rally 4 where they try and milk extra cash out of you by having you call a premium phone line to get unlock codes.