Militarily, he was exceptional. He professionalized the Parliamentary forces he commanded, and was one of two, IMHO, great commanders of the English Civil War [the other being Prince Ruprecht]. That being said, he overthrew the Parliament he went to war for, and established a dictatorship. As was posted above, what he did to the Irish wasn't pretty. He was also one of the driving forces behind the deicide of Charles I. A mixed bag at best.