Today many of Sun Tzu quotas are floating around mixed with Shakespeare and the Bible. And as with many of the quote they are out of context and misleading. The quoter usually thinks repeating the word is some sort of magic.
It is easy with hindsight and a closer look at the future to dismiss Sun Tzu as his practical tactical knowledge is of a time and place long gone. He spends a lot of time on the use of weapons and information gathering techniques of the time. This can be interesting in a historical context; other wise it is quite amusing.
Oh yes those grains of truth I mentioned, well they may sound like clichés but they are still viable. "Know your enemy and know yourself". Others are just practical sense and statistical outcomes that you learn in any military training. I could go through the list, but again that is why you buy the book.
Now just as you decide that the book is outdated for any practical purposes today we have artillery and now stealth and precision, the reminder that "no two wars are alike" and "it is flexibility that makes a difference" is being shown today to still be true. Even in today's wars there is a need for good intelligence and deception. We put a lot of time and energy into Psy-Ops. Sun Tzu shows the advantage in specialized units and crack troops.
I have spent several years in the military and in business and can say this book is a nice addition to history, otherwise of very little value to today's world for war.
As a Project Manager I can tell you that this book has little value to today's business. Projects are about efficiency, cooperation, and ROI not war.