I switched thanks to the influence of my Wife about 5 years ago. I have not missed PhotoShop! I did not use it on Windows, mainly because I did not want to spend so much money on a "drawing" application. For most of my Windows time I used CorelDraw and its tools to do the trick.
Most of those who quote PhotoShop, regard GIMP as a tool that has a cumbersome Interface. This argument is not valid if one uses the "hacked" version of GIMP called GIMPshop, that rearanges all menu entries to look like those of PhotoShop. But, I again don't see this as necessary. Having spend 5 years with Linux, I have gotten to know the normal GIMP and when I tried GIMPshop today, let me tell you how cumbersome it was. Why are all the tools (color etc.) under the menu entry Window? Seems to me like having to click on Start to turn off your computer (sarcasm).
Without wanting to bore you with more details, the flags on our homepage where made using GIMP from Flick photos (all Creative Commons licenced, so the my creation is too). As you can see, even a moderatly computer savy person can use GIMP to create fine works.
Current mood: 🙂
Update 07.08.2006: The Guru SuSE RPM archive has GIMPshop RPMs.
A lot of people tend to quote the lack of PhotoShop as a reason why they would not switch from Windows to Linux.