I got about 70% of the way on Realistic mode and gave up. Too many parts were overly-frustrating.
I will say that the 70% of it that I went through was totally fun. It started out as an experiment doing just the first level on realistic, and it was so fun I tried to do the whole game on Realistic. You have to be MUCH more patient, use the binoc's more often, and be more efficient with ammo. But like I said, it was fun.