Fits snuggly in your (well, my) hand, has easy to use joysticks, due to both shape and position, and while having the same number of buttons as the duelshock, puts the L2 and R2 buttons in the 'trigger' posision, meaning that you can actually use them conveniently.
The big drawback of having the joysticks in a more convenient position is that the D-pad has been pushed out of the way, making it somewhat uncomfortable for me to use as a movement control (which can be an issue in arcade games).
As I said, I love my PS2, but I was never a huge fan of the duelshock. They basically took a SNES controler, put two knobbles on it for your palms then, when they realised they'd need joysticks, simply stuck them on the bottom rather than change the design.
It's more than good enough, but having used somthing better I find it hard to go back without noticing such (which, again, was part of the reason I hated the huge bearclaw Xbox controlers, even the smaller ones were pretty horrible, but then, I never liked the dreamcast controlers either).
Of course, all the above is just to my own hands, you might find yourself of a different opinion, but the first time I got to play on a 360 (having spent so much time on my PS2) the controler really stood out.