The Art of War is one of the foundational works of strategic thinking. Essentially it is a treatise on the principles of warfare based on ancient Chinese military history.
The growth in academic and business study of strategy has given this book hugely increased popularity over the last twenty years. However, strategic principles are often extracted and applied haphazardly, based on what the reader was looking for.
It is important to keep this book in its context. Some of the principles - such as 'to win without fighting is the highest achievement' are obviously of general applicability. Others, such as 'when plumes of dust are seen, chariots are approaching' are clearly of little relevance to modern business. However, when faced with 'in a forced march of 100 li the commander will fall', you may decide that there is a useful application or you may not.
Likewise, you may find it entertaining to quote Sun Tzu to your colleagues, and it may give a flavour of authority to your strategy proposals. If you do so, though, you ought to recognise that your application is your own interpretation, not something intrinsic to the text.
If you are reading this book because you want to learn about Chinese military history, or because you want to understand the way early strategists thought, I believe you will find it enjoyable and rewarding.
On the other hand, if you are using this book as a way of developing your own strategic skills, I would recommend that you read it in conjunction with Mintzberg, Ahlstrand and Lampel's 'Strategy Safari', and particularly note the critique of the Positioning school of strategy. The dangers are all too great of finding in Sun Tzu echoes of ideas that you already hold, and then imagining that these are supported by ancient authority.