So actually, grad school classes may be hard, but unless you're really lazy, you're going to get a good grade in the end, since professors only give out As and Bs. That's half of what people mean when they tell you about how you shouldn't care about grades -- you're going to get that top grade whether you care that much or not.
For instance, I discovered today that my 87% on the ghastly problem sets in my quantum mechanics course was good enough, to my surprise, to earn a grade of A (we had no exams). Sure, some people got overwhelmed and didn't even seriously try to finished 87% of the problems, but now I think I actually believe it when I'm told that if you do all the work you're going to get an A.
Just for kicks (you can tell I've finished all my work for the semester already)...
Quantum mechanics problem set grade over time (aka proof that I have gotten lazy, but apparently not too lazy!):