It all depends. How extensively do you use your current version? Does it meet all your needs? CS has a lot of cool new features.
Better file browser, faster.
It now handles RAW files from your digital camera.
It now has real "Text on a Path"
It has shadow and highlight correction. You can use Imageready to create Flash animations.
It has a great stitch feature where you can take 2 scans of an oversize image and it will automatically align the images into one. We use CS (Mac version, OS-X Jaguar) at my girlfriend's print shop and I have played with it and I am impressed. For what they do at the print shop, it is great.
For my personal use, I do not see any new features that are screaming for me to go out and upgrade. For text on a path I use CorelDraw or Illustrator, then import that graphic into PS6.0.
I am now tempted to pick up a PS7.0 copy for home now that you can find good bargains on ebay. However, even 5.5 would suffice for 99% of my photoshopping!
When in doubt, deny all terms and defnitions.