Why you should read this book:
1. It's not to heavy (thought it does make you think a lot). I have read a couple of puritan book of the 17th century and they are filled with great stuff but because you read them in the original English it's hard going, but this book translated from the Latin is much more readable. Although the book is v. long it is not as hard as you think it is - trust me.
2. It is nice to read a good theologian not setting out purely with the aim of defending the doctrines his own denomination has been teaching for centuries. Although is influenced by tradition he is not as obsessed by it as some Protestants today. He does suffer slightly sometimes, i.e. has some wrong ideas about minor points (e.g. the ancient church on confirmation), because he is not just re-plowing a furrow that has be furrowed a thousand times, but these slips are usually picked up in the notes. It's so refreshing.
3. He really, really cares about the truth. Yes he does sometimes call his opponents "dogs" and "swine" which is less acceptable now than it once was, but he calls them that because he is angry because he sees heretics catching Christians in their nets, are you not upset when you see that?
Earlier reviewers have called him a tyrant because he used his limited power (he wasn't even a citizen of Geneva) to try to stop people sinning as much. Sometimes he went a bit overboard but at least he cared.
4. He uses the church fathers a lot more than anyone else I've ever read. He had read so much compared to now. I have heard that he worked very hard, 4hrs sleep, into an early grave etc, and it's not hard to see what he did. He was a full time pastor and yet had read all these books. Scripture is infinitely better than the fathers, but Calvin was concerned about the Catholics and he uses Augustine etc to show the Catholics of then and now that their beloved fathers would have hated the RC church post-500ish. You won't get that much elsewhere.
5. His chapters on providence and man's sinfulness. People think this book is all about predestination to salvation, but it doesn't really have a central theme like that. But essential to your understanding of election is God's providence and our depravity and Calvin gives these the right weight and makes so much so clear. However overriding all his writing on election and everything else is that we should try to understand as much as the bible tells us but go no further. He was, it seems to me (<I can't see his heart like God can), really humble before God and his word.
The fact that this review is so badly written should prove to you that I am not an eminent scholar, just a lowly maths student, and so this book is easy enough for most to read. Don't bother with an abridged version spend the rather large amount of money and get this book - it is worth it. If you want a big book mainly for reference get Systematic Theology by Louis Berkhof which though not perfect (no book by man ever will be) has more scripture references and less human writing.
However, if there was one book other than the Bible with me on a desert island it would be this one. No other human author has ever been as edifying for me. He helped me realize for the first time since I had started calling myself a Christian a few months earlier that I was saved totally by grace and am myself the most vile creature on earth when you realize God's holiness (read Hopeful's story in Pilgrim's Progress that's me). This book (would you believe it a 16th century work) truly drove me to my knees. Buy it! Sorry for rambling.