I just got C4 yesterday, and the new IK is definitely a vast improvement. I'm not into character animation, but I whipped up a chain (literally, a chain made of linked rings) and used the IK tool to make it work, and it was really nice! The links intersected each other, of course, because I hadn't set up contstraints, but when I dragged around the end of the chain it moved very nicely without the hangups and freakouts I'd sen in C3. I'm not much into character animation, or at least I wasn't with C3, but I just may give it a shot now.