I just upgraded from PS7 to CS3 a couple of days ago. Can't give an overall reaction yet, but I will say that the upgrade process was simple and easy (I got the DVD version from Amazon, so I can't say about the electronic dl version). 

CS3 starts up fast and doesn't seem to run a bit slower or hog more resources (the fear of that was a big reason for me not upgrading to CS2). Outside of having to reset all my preferences, the transition has been seamless so far.