Rothbard's opus will teach you about the ethics of a free, nonviolent society, and how this society will prosper. It also does a good job of demolishing the concept interpersonal utility comparisons, which will be a great thorn in the side of those who advocate "welfare" spending. It also shows that, unlike most followers of Marx unquestioningly accept, the capitalist is productive.
Starting with a few easily understandable and self-evident principles governing human action, Rothbard leads the reader step by logical step into a deep and comprehensive understanding of all the key areas of economic thought - supply and demand, price, economising between scarce resources, savings and investment, money, time preference and the rate of interest, monopolies, cartels and unions, market "failures", and the issues surrounding government intervention in the market (phew!). The beauty of this is that at no point does the subject matter become obtuse or incomprehensible - any reasonably informed and intelligent layman can follow the train of thought from beginning to end.
In complete contrast to the dry professorial meanderings of almost all economics textbooks, Rothbard's lucidity of thought and infectious enthusiasm for his subject matter practically bursts off the page. Almost every sentence is concise, clear, and directly to the point. Complicated subjects are explained in clear English, and often seasoned with wry humour or a jab of razor sharp wit. What a breath of fresh air, and a pure pleasure to read!
Of course, as a follower of the Austrian school of economics, Rothbard is vehemently anti-state, and makes no attempt to hide the fact. "Right-wingers", and even people who consider themselves staunch libertarians, may find themselves shocked at exactly how far Rothbard takes his train of thought. And Keynesians would be advised to pour themselves a stiff brandy before sitting down to read - this man takes no prisoners!
However, regardless of your political beliefs, I believe this to be essential reading. If you are an advocate of liberty and free markets, you will find yourself inspired by this impassioned and erudite defence of everything you hold dear. If you are a hardcore statist, liberal, egalitarian or Keynesian, then you will find your beliefs challenged and shocked to the core. Either way, you cannot fail to increase your understanding of the economic and political world, or be anything other than impressed at the towering intellectual achievement that this work represents (think of it as the free market version of Keynes' "General Theory"). If you are a layman, then you will finish the book with the realisation that this whole economics business can actually make sense after all (make sure to read Keynes, Friedman, and Samuelson to get the opposing mainstream views).
"Man, Economy, and State" was one of the 20th century's first attempts (Hazlitt possibly excepted) to take economics out of the ivory towers and put it back into the field of intelligent mainstream debate. Rothbard has built on the great achievements of Ricardo, Bastiat, and especially von Mises, and produced something at least as important, if not greater. Of course, being a fiery indictment of practically every mainstream school of economic (and political) thought, it was practically ignored by academia, and it is only through the passage of time that its reputation has spread, mainly by word of mouth.
I will admit to being heavily biased in favour of this book. My profession involves unfettered capitalism in the financial markets. My political beliefs are (mostly) libertarian. I consider Rothbard to have successfully engaged in a complete and utter demolition of Keynesian economic theory, and at least 90% of all cases of state intervention in the free market. But none of that matters - just as I read Keynes and Kaldor, and even ( shock horror) Marx, Chomsky and Pilger to understand the main opposition to my views, so others should do the same. You cannot coherently hold strong views without understanding the main criticisms against them.
Buy this book, and you will come away much wiser. At the very least, no one will be able to say that you "simply don't understand" economics. You might even have a few laughs along the way!