I can proudly say that I have read the Institutes of the Christian Religion cover to cover at least twice. And I have read other of Calvin's writings. And I have read with real benefit books about Calvin by Reformed theologians. During my 26 years of teaching historical theology, I have always required my students to study Calvin and, in high-level courses, to read portions of the Institutes.
While I am generally opposed to Calvin and his theology, I find his writing stimulating and thought-provoking, and I largely agree with his doctrines of the knowledge of God, revelation, Scripture, and humanity, including sin. And I find his accounts of the person of Jesus Christ and the Trinity completely biblical and orthodox, if not particularly interesting.
One area of Calvin's theology with which I especially agree is his doctrine of Word and Spirit in the Institutes. Although much of the material there echoes Luther, Calvin puts it in a particularly lucid and incisive manner.
Many Calvinists and charismatics will be surprised to read these passages, because in them Calvin rests all true knowledge of God on the inner testimony of the Holy Spirit. In my experience, too many Calvinists talk too little of the Holy Spirit, while too many Pentecostals and charismatics know little to nothing about Calvin's emphasis on the third person of the Trinity.
According to Calvin, all positive knowledge of God derives from Scripture, and all truth claims about God must be judged by Scripture. But the authority of Scripture rests on the Holy Spirit, who inspired it, and a valid understanding of Scripture rests on the enlightenment of the Holy Spirit.
Calvin was no charismatic, but he was closer to it than some Reformed people readily admit. At least ...