I think I am moving to CS4 for the MAC, I have CS3 for windows, but when I get the new macbook pro and with my Imac I want to take advantage of the new features using the GPU which in my mind is enough to upgrade anyway..
here are some of thing I have liked:
Content aware scalling,
Ability to rotate the canvas in real time, zooming in and out with out losing quality, the gpu can redraw quicker so it is a smooth zoom in and out,
being able to throw the picture with the hand tool and pan around the image fluidly without losing any quality,
when you zoom in over 500% you get pixels mapped out in squares so you can easily grab colors and such,
The new adjustments panel, lets you add adjustment layers easier then before.
You can use a button combo now and drag to the right or left to change the brush size which is cool another combo lets you adjust softness the same way
I like the tab layouts, and the other thing is now for mac the window is one piece not floating on the desktop, more familiar to windows users switching.