The GIMP is absolutely amazing. It has been around for many years now and is a very mature and feature rich program. It has so many features and possible uses that if you can do something in photoshop, you can do it in the GIMP. Oh yeah one more thing, the GIMP is free, unlike photoshop.
I agree, GIMP is a very robust program for photo editing or starting from scratch. If you were born and bred on Photoshop x.x, it may seemed stripped down or simplified at first. However with a little time and reconditiong, you will find the features that the $600 (or whatever it may be now) competitor provides. It's also friendly in multiple OS's. I've used it heavily on both M$ and Linux and they are both rock solid. You can save your menu settings to give it a more conventional Photoshop look & feel, if you aren't comfortable with the "right-click" method. It's a sweet program and there are tons of filters out there. -or make your own! It's also digital pen/tablet friendly in both OS environments.
Unfortunately, on Mac OS X, Gimp requires X11 -- which adds overhead and leaves you with the Unix interface. Hopefully, the GUI will be Mac-ified and dropped into Carbon, Cocoa or Java in the future to run as a native app.
Comment posting temporarily disabled due to abuse (spamming attempts)