My brother came across a Tzu/Machiavelli/Clausewitz compilation in a book store one summer.
I was hooked on Total War games and had a lot of time on my hands, so I read all three books in less than 2 days...
On War by Clausewitz was the most compelling to me, but 18th-19th century warfare has always intriguied me.
Machiavelli's book, which I believe was also called "The Art of War" didn't appeal to me very much, atleast compared to the others. Mainly because I don't like his political ideal. He may be very right, but that doesn't mean I have to like it.
The original "Art of War" is wonderful in that it's not just about warfare, but can, as mentioned by others, also be applied in so many situations in modern society. The man was a genius and it'd hurt noone if rulers followed his ideal more closely.