People who know me are aware how big a fan I am of the venerable application that is Photoshop/Imageready. After having shifted to Linux(post on that pending), I decided to have a look at GIMP today, based on a tip-off by a FLOSS buddy of mine. My findings are more or less concordant with the rest of the world (from what I found on google), but I'd like to add a few things to what everyone else is saying. I'm sure there are more people who're using the Fedora Core 1 distro with GNOME and are considering the GIMP as a serious replacement for Photoshop, so here's a few things that'll help them:

First of all, get the 2.0 version. Fedora ships with 1.x, and there's HUGE advancement in the UI, etc. Trust me, it's worth the download. Note that Fedora doesn't gracefully upgrade gimp, so use the instructions given at fedoraNews for this.
Once you have things installed, start the app. It'll look pretty, but not quite. This is because you're used to the lean, mean Photoshop UI, so we'll need to get that customized for you. So go to the menu on the tools bar (ignore the weirdness of menubars everywhere) and choose File > Preferences. Select "Interface", and set the "Select Theme" option in that to small. This will choose a smaller font for the GIMP UI, and make things look pretty.
Now for the tools bar. Well, it looks a little disorganized, but if you resize the window so that there's 3 tool buttons per row, the layout looks pretty decent, if you ask me.
Now try opening an image. The cool thing is that I could open most .PSD files - a great benefit. A lot of people are complaining about the menu on each image. Actually you can easily fool yourself into a convenient UI - just maximize each image window. If you switch between windows now, it'll look just like the Photoshop menu. All you got to remember here is that there's another set of menus in the toolbar's menu bar.
Using images in maximized mode will irritate you since the tool bar and other GIMP windows will keep dissapearing under the maximized window. So we need "always-on-top", but can't have it because there's no option for that in the window right-click? Fret not, there is hope. Just like this page says, Metacity (i.e. Fedora's default GNOME window manager) in GNOME 2.4 onwards does have always-on-top. And since FC1 has GNOME 2.4, all you need to do is go the Fedora Menu and choose : Applications > System Tools > More System Tools > Configuration Editor. Now navigate to apps/metacity/window_keybindings and assign the toggle_above keybinding a key combination. I set it to <Alt>F11. Now close the conf editor and run GIMP again - now you can press Alt+F11 to set any window to always-on-top. Do that for the tools and other floating windows.
There's a thing called "Layer Effects" in Photoshop. GIMP doesn't have an equivalent, and I miss them a lot. Plugins to the rescue, go install this one. It's not quite the real thing, but does quite well.
The last thing for today is fonts. This is how I installed my entire collection of TTF fonts from windows: open two nautilus windows, one listing the folder that contains the fonts; the other pointing to fonts:///. Now select all the TTF files you want to install, and while keeping Ctrl pressed, drag and drop the fonts to the fonts:/// window. Now refresh the fonts list in GIMP - yay! We have SilkScreen on the GIMP! Now restart X (easiest done be logging out and logging back in), and you have all the fonts installed for the entire of your desktop.

That's all for today, folks! Happy GIMPing!