Having used a lot of program suites in college, from InDesign to Apple Final Cut to Photoshop to Adobe Premiere and stuff I can't even remember....yeah. It's crazy how some of these programs mix, match and overlap. I've used professional photo editors that can't, for example, draw a simple black line to the image. Because it wasn't designed for it. GIMP may allow you to tweak levels in the image, but it doesn't have even half of the tools, effects, filters and stuff that PS or an actual photo editing program has. I remember firing up GIMP to do work in school one day, and my photo professor walked by and said "what the hell is that?" I told him what it was, showed him how I was manipulating color levels through GIMP the same as you do in PS...and he was like "Why are you using that instead of Photoshop on the macs?"
My answer was simple. "Because I don't need to fire up Photoshop on the schools' over taxed mac to do simple stuff, when I can fire up GIMP in flash on the PC and have it done in under 2 minutes."
I can't say I've used everything by far, but GIMP just seems to hit this sweet middle spot between photo editor, paint program and drafting. It probably falls short in each department because it's trying to do them all, but if you're good you can create stuff that easily could pass for having been done in PS.
And since we're on the subject, is there any plug-ins or downloads for paint and draw tool shapes for GIMP? When you're working on a really small piece of bit art, the default shapes for GIMPs tools are kind of annoying and there isn't a great variety. I can always work smaller, but it'd be nice to have a square instead of a circle sometimes.