For a long time, I thought the Grunfeld was a ridiculous defense. Moving your knight three times, giving white that big pawn center before you've even developed a single piece... terrible idea, right? Then, after years of playing the Nimzo-Indian, and playing around with several other d4 defenses, I gave the Grunfeld a try. And I *really* liked it.
I find the Grunfeld to consistently create lively, complex games. I would also say the games are more "open" than any other defense to 1.d4, though in a positional sense rather than classical, if that makes sense. You definitely need a good sense of how to attack a pawn center, and a solid understanding of the tactical themes to play it successfully. You can come out of it in great position, or get squashed very horribly.
There is some theory to know, and the exchange lines can be particularly sharp (in addition to the most common). At USCF Class C, though, I would venture that your opponents are not going to be accustomed to seeing it. Give it a try... you may find that the tactical considerations are a bit much to manage, or you may find that you like it.
I've been playing the KID the entire time I have been playing chess. I went to the Benko at one time, but gave that up real quick becasue I don't like the declined version. I really like playing different openings so I thought it might be good to find another one against d4. Recently I have been messing around with the Tango, but I don't like white playing Nf3. I refuse to play d5, I just don't like symmetrical openings and the amount of theory. I also like to play aggresively as you could figure out by my name, and so the Grunfeld sort of seems fitting.
So I've looked at some Grünfeld games, and I think it is ok. But I have heard some negative things about it, like you need to memorize a lot of theory. Is this true for, say, USCF C section? And what are the most dangerous, and common lines for black to face? Thanks. I'm willing to play 1.d4 d5, but not very excited about it. I want something that is interesting.