I will add my experience with a new install of PS CS4. I installed the CS4 Design Premium suite on my computer that already had the CS3 suite installed as well as numerous other programs, all of which work well. The installation completed without problems. All comments are preliminary representing a first pass through the application.
My system contains the following items:
Asus P5Q-E board
Intel Core2 Duo E8400 3.0 GHz
8 GB RAM
320 GB C drive for sys and CS4
50 GB partition on a 2nd physical drive for scratch
Intuos 3 tablet not yet tested, using mouse for initial eval.
Vista Ultimate 64-bit , Aero enabled
Photoshop CS4 (64 bit), Open GL enabled
No plugins
Nvidia GeForce 9600GT 512 MB
Driver 7.15.11.7813 (Sept 2008, not the latest)
Eizo CG222W display 1680 x 1050
After verifying that PS performed its elementary operations, I took a brief look at brush lag and I experienced some at large brush sizes. I then kicked the tires of some of the other apps in CS4 to verify their basic operation and all was well with them. I then went to Bridge and set many preferences, did some e-mail and other things as well. When I reopened PS to check it out in more detail, the lag I had seen previously was gone or much reduced. I cannot explain why.
Currently, all PS tools are working OK and all the new CS4 navigation schemes behave well. There is, however, some occasional, unpredictable behavior with the tool tips and the history states. I've now determined that the brush lag is real, but only for large brush sizes. While that sounds like bad news, the exact same test using PS CS3 on the same system yields comparable results, maybe a tiny bit less lag, but within say 20% of the CS4 performance. The comparison was made on a 4000 x 3000 image and quickly moving the rough round brush of diameter 1000 pixels. I've also seen stuttering cursor movement (only on CS4) when a brush diameter is approximately = or > 20% of the image width, but this effect cannot be consistently produced and may be a pathological finding.
My bottom line is that PS CS4 is working on the above described computer and working well enough to get work done and not force me back to CS3. It does have some defects and most likely I will find more. Tablet issues have not been addressed yet. Additional effort to give a real review needs to be done, but with all the reported CS4 problems I thought my initial experience may be useful input. It demonstrates (at least) that with the right (unknown) configuration details the above hardware and software produce acceptable, if not yet great, results.
Paulo