I think its a shame, and a pain in the neck that Adobe haven't developed a better method to move up, or upgrade between versions. Everytime a new version comes out, I have to manually go through and copy all my presets, gradients, brushes, actions, workspaces & keyboard shortcuts, the list goes on... And I wont even start with the nightmare that is re-installing plug-ins.
For a very professional application that is used largely by a professional market, it is a very polished product now, however they have done nothing over the years to make it easy to upgrade to a new version.
I have upgraded to CS4 now, but still use CS3 80% of the time, as I don't have the time to copy over everything I mentioned above, and I'm still waiting for lot of plug-ins that see daily use to be compatible with CS4.
I do like CS4 though, the new interface is very slick, and once you have figured a few things out and created a new workspace, it is good to use.
Performance wise there is a noticable difference between CS3>CS4, it handles everything smoother and with speed. My main reason for upgrading so early were the improvements made to Bridge, as I use this daily for image management, and as in Photoshop, Bridge is now a joy to use.