For a lot of folk, CS4 seems to run just fine, with performance in most areas similar to CS3 (some areas better, some worse). There are however some real performance issues for others. Unfortunately, I'm in the latter category.
The biggest and most noticeable problem (that I've encountered thus far) is that of stuttering and lag when using any brushes or the clone/heal tools. Even the smallest, simplest brush will lag quite a way behind the cursor making it difficult to use, with the heal brush being to the point of almost unusable entirely.
After spending a few hours looking through the Adobe (and other) forums for a solution, it appears there isn't one, or indeed any explanation to exactly what the cause of the problem is at this stage. The problem is appearing on all sorts of hardware/software/OS combinations.
I've noticed one Adobe Dev on there who seems to be looking into it, and he has provided a couple of registry tweaks which in my case have certainly helped. Of course you use these at your own risk, but they work fine as I've tested and are reversible with the optional reg tweaks in the download. More info on the two registry edits is available HERE. In my case the AllowOLDGPU reg edit did the trick. Even though my vid card isn't old and is of course on the list of tested GPUs working with CS4. The reg tweaks can be downloaded from the Adobe site HERE.
After applying the patches, the brushes are much more responsive and the lag has been greatly reduced, still not to CS3 performance levels, but at least to a level where CS4 is now usable.
Another problem I encountered was the screen not refreshing after applying anything to an image. If I used the clone tool for example, the screen would not be refreshed with my changes until I clicked onto another tool or used the same tool again. I discovered that this was due to a couple of Nik Software plugins I had installed. A quick update to the latest versions of Viveza and Sharpener Pro fixed that problem.
All-in-all, a rather frustrating exercise, though I'm at least able to now use it satisfactorily. Here's hoping Adobe actually pull their fingers out and address this problem with a patch before long, though I fear it'll be CS5 until anything is done about it.