I use photoshop on a daily basis... I own CS3 for my personal computer, and I have/use CS4 at work and school... and I prefer cs3 hands down to cs4. cs4 honestly did not change very many things beyond renaming and some layout features. It may have some shiny 3d related tools and features, but in all honesty, I have NEVER had the need to use them (And I'm serious when I say I use PS every single day for school and work).
I prefer CS3 mostly for layout and simplicity. CS2 you get handicapped a few tools, which is why I upgraded to CS3. CS4, imo, is just more unnecessary frills.
Also, I should note that I've actually PAID for my Adobe software. If you're just going to be a fucking leech, get CS4. But, if you're looking to be cost effective, CS3 will deliver all the power and features that you will most likely ever need for a reduced cost and (in my experience) reduced toll on your computer.