Many of the points in Sun Tzu's are very relevant even in modern warfare.
Here are a few examples of quotes that can never die:
Chapter 2.
5. Thus, though we have heard of stupid haste in war, cleverness has never
been seen associated with long delays.
6. There is no instance of a country having benefited from prolonged warfare.
...
19. In war, then, let your great object be victory, not lengthy campaigns.
Sun Tzu recognizes that war should end as soon as possible, it is still true today. American invasions to Iraq costs the nation billions that wouldn't have been incurred if the war progressed swiftly.
Chapter 2.
9. Bring war material with you from home, but forage on the enemy. Thus the
army will have food enough for its needs.
American "foraged" oil from Iraq.
Now, there are also parts that are outdated in today's modern world, such as this examples about chariot fights:
Chapter 2.
17. Therefore in chariot fighting, when ten or more chariots have been
taken, those should be rewarded who took the first. Our own flags should
be substituted for those of the enemy, and the chariots mingled and used
in conjunction with ours. The captured soldiers should be kindly treated and
kept.
However, if you fete out the chariots, there are the abstract notion of rewarding soldiers, symbolic flag gestures, and how to treat captured enemies; all are still relevant in modern warfare.
Some are also outdated due to technological advances, sieging a walled city has been much easier since the invention of cannons and modern missiles makes sieges more reasonable than ever:
Chapter 3.
4. The rule is, not to besiege walled cities if it can possibly be avoided.
The preparation of mantlets, movable shelters, and various implements of
war, will take up three whole months; and the piling up of mounds over
against the walls will take three months more.
Some are less relevant now that the primary means of warfare is no longer foot soldiers/infantries. But infantries still are an important part of modern warfare.
Chapter 6.
1. Sun Tzu said: Whoever is first in the field and awaits the coming of the
enemy, will be fresh for the fight; whoever is second in the field and has
to hasten to battle will arrive exhausted.
We can't just send a bunch of missiles and hope to win the war.
On the other hand, some strategems are becoming even more important in modern warfare:
Chapter 13.
4. Thus, what enables the wise sovereign and the good general to strike and
conquer, and achieve things beyond the reach of ordinary men, is foreknowledge.
5. Now this foreknowledge cannot be elicited from spirits; it cannot be
obtained inductively from experience, nor by any deductive calculation.
6. Knowledge of the enemy’s dispositions can only be obtained from other men.
Spying plays a much more important roles in modern warfare than it ever were.
Sun Tzu recognizes the importance of being able to read the "ground" and how to best handle them:
Chapter 11.
46. Therefore, on dispersive ground, I would inspire my men with unity of purpose.
On facile ground, I would see that there is close connection between all parts
of my army.
47. On contentious ground, I would hurry up my rear.
48. On open ground, I would keep a vigilant eye on my defenses. On ground of
intersecting highways, I would consolidate my alliances.
49. On serious ground, I would try to ensure a continuous stream of supplies. On
difficult ground, I would keep pushing on along the road.
50. On hemmed-in ground, I would block any way of retreat. On desperate ground, I
would proclaim to my soldiers the hopelessness of saving their lives.
While some values might have changed since the time of Sun Tzu, there are many general directions that are ageless.
