My reading of economics has lead me to believe that he was the greatest economist in the past one hundred years.
I would be not be likely to read Von Mises' treatises any more than I would be likely to read the writings of a creationist. He was not a great economist in any sense. His claim to fame, from what I have read, was his wonderful observation that human behaviors are just so damned unpredictable hard to understand that Economics cannot be subjected to empirical scientific scrutiny. It arose from an observation of exactly what he espoused in his religion. That is, the late 19th and early 20th century was characterized by huge boom-bust cycles caused by those famous "business cycles" that he so trusted. These trends persisted through the period before there was a central bank and after. Hard to blame them on the central bank.
As far as being a worthy competitor to Keynesian economics, Keynes' views got no traction until 1930. Hardly good timing for a coherent comparison. Keynes had a tough time convincing FDR that deficit spending was the remedy for the Great Depression, and like Obama, Roosevelt waited until WWII forced him into substantial deficit spending. Keynes' views got plenty of vindication thereafter. Mises' religion has never been vindicated in any country or any economy in the history of the world.