I just bought a Mac and installed the CS4 Trial. I am a professional artist and have used Photoshop for going on 7 years. When I make industrial design style sketches, I use shift-click to create straight lines in any direction. I have been using a Wacom Cintiq with a PC for a few years now. I recently bought a new Mac Pro, and I find that Photoshop does not react to input the same as on a PC.
Here is the problem. On a PC, if using a brush with size and/or opacity set to pressure sensitivity, the first and second click's pressure will register correctly. You will be rewarded with a fat, thin, dark, or light line according to your input. On the Mac platform, the second click is only registered as the lightest of taps, no matter how hard you press down. This results in either no line, or a cone like line after multiple clicks while holding down shift. I get the feeling that Photoshop may not be registering even the first click correctly. If it did, you would always get a conical line. Instead, you get no line.
I tested this by making a brush with only size set to pressure sensitivity, and minimum size set to anything above 0%. This will result in a line being drawn. Hence, the program treating the clicks as though they were made with the slightest of pressure.
This is unacceptable. How can a program behave so differently from one platform to the next?