Very difficult at times, but equally rewarding
I will agree with other reviewers that on Hard level, the enemy has some kind of supernatural powers: they can kill you with a machine gun from about 100 yards and they know where you are instantly, no matter how careful you are (or how many times you reload and try different things). But that doesn't ruin Hard level - you just have to realize that is the way the game works. You can stay behind cover and run from cover to cover, and take the enemy out from very long distance. It may take away from the realism just a bit, but it certainly makes the game much more challenging on a second play through, and I think that more than makes up for it.
I'd try this game on easy difficulty first, and after you have the hang of it, you will want to move up to Hard. It's a very fun game and has excellent replay value. There's something about blowing an enemies head open, like Gallagher smashes a watermelon, that is extremely satisfying.