I've upgraded each time from CS to CS3, but only because the upgrade gave me the specific additional capabilities that broke some bottleneck in my workflow, like enabling Liquify to handle 16-bit files. That little update in CS3 allows me to make Liquify changes right up front where they should be, rather than at the tail end of editing.
But I haven't seen anything yet in CS4 that would be worth upgrading. At this point, the learning curve of the GUI would just slow me down for a while without giving me any new capabilities at the back end of it.
I'm not going to a 64-bit system before CS5 (I depend on too many applications that are still 32-bit), so no reason to upgrade for that.
The big Bridge upgrade happened in CS3, and while I'd be interested in an improvement...I'm not that interested. There are some new video improvements, too, but it sounds like I'd need to update all three of the video cards I'm using now...so I can live with what I've got now until CS5 and a 64-bit system.