Who is Matt Stoller? Is he a known quantity?

Stoller is an FDR-ish political progressive who focuses primarily on monopolies and antitrust enforcement. He's the author of "Goliath: The 100-Year War Between Monopoly Power and Democracy": https://www.goodreads.com/book/show/40538538-goliath

Answer above is good, but yes he's a known quantity. I am a political moderate and definitely don't agree with everything he says, but I find his POV (and knowledge of antitrust history) to be a valuable perspective.