I’m a bit underwhelmed at the moment, can’t see anything “must have” appearing here. The new palettes and layout methods do look promising (though require getting used to).
I’m assuming Adobe have left out a few features they intend putting into the release candidate because as they stand CS3 running native on Intel Macs looks like the only compelling reason to buy.
Quicker start up times on Windows PCs is nice but not exactly compelling. The new Bridge crashes too often for me to use it much.
I would advise clients on Powerpc macs or windows XP that they are not missing much at the moment though you can download and install the beta and run it in parallel with CS2 to see for yourself.