I'm a web dev/designer, currently using Photoshop CS3. I don't know CS5, so not sure what I'm missing - would upgrading to CS5 noticeably improve my work or workflow (assuming I learn how to use it)?
Do the top web designers consider CS5 necessary? If you mock up sites in CSS instead of Photoshop, do you use CS5 for other important things, or are other programs (or older versions of Photoshop) just as good?
I'm currently using CS5, but it's largely because of some of the small addition features with smart objects, workflow, and layer styles that annoyed me with CS3. It's one of those things where you have to work with a program for a long time before you notice these small upgrades that really help the day-to-day. I think you're fine with CS3.
The idea that GIMP is more than useful is... debatable. I would say that every notable designer I know in NYC/SF uses Photoshop. Like it or not, it is the industry standard. And while I do sometimes have a love-hate relationship with the program (specifically because I want more stability instead of new features), GIMP is still far behind with regards to it's intuitiveness and penetration.
Now that I've migrated to a startup, I use Photoshop for quick initial concepts before coding scss. Photoshop is pretty solid for making elements or re-using templated elements made from other designers and photo-editing.