He is (or pretends to be) pretty left wing and wants (or pretends to want to) bring the Social Democrats back from the center to Social Democracy. That makes him "worst of the worst" for someone of REDY's political leanings.
He's currently our best hope to get rid of Merkel and Schäuble and the austerity fetish, but he's pro EU and German, which makes him "vermin".
I myself am pretty sceptical. SPD in campaign mode os usually more left-wing than SPD in government mode, but at least they aren't outright dismissing the possibility of a coalition with our actual leftist party. That might be one of the reasons that the SPD's poll numbers are skyrocketing now.
A probably more important reason is that Schulz hasn't been a name in German politics for a while, so he has the veneer of an outsider despite his long political carreer. That gives him a quasi-populist appeal without the obvious incompetence or the hate- and fearmongering. He might become chancellor simply because the German electorate isn't sick of him.
I did some digging into Schulz. Personally I am pro-EU and I tend to lean anti-austerity. But aside from those, I can't say I agree with the man much. His promises regarding the economy seem like nothing more than empty vote-buying (to be fair, this is something every politician does, and not just him). He also seems pretty pro-immigration, which, while I disagree with, isn't my problem unless he tries to force his views on all of us through the EU. Furthermore, I have some doubts about his competence. Perhaps I'm wrong about this, we'll see if he ever gets elected.
To sum it up, if I were German and I had to choose between those two based on the information I currently have, I'd vote for Merkel.