I think Starlin wrote Superman perfectly in this last issue: hella powerful, noble, and tossing himself into a cause that he has no business being in.
Superman acquited himself well, but ultimately didn't affect the outcome (except maybe to buy Darkseid a little more time). The Infinity Man was clearly not going all out on him... he's shouting about how Superman shouldn't interfere and as long as he doesn't his life would be spared. He got some good hits in, but eventually was pushed out of the way because he's not the center of the mini.
What's with all this arguing about how well he did?