Best. Error. Message. Ever!
A painful confession, don't laugh!
I have to come clean and admit I never really got past the dabbling stage in Adobe Photoshop, I grew up using and loving Fireworks (back when it was Macromedia). For my own uses with my crappy old Lumix camera it did everything I needed it to in a fraction of the hard drive space and with a much cleaner interface; people complain about The Gimp's interface but I think much of it comes down to what you're used to, I thought the Photoshop interface was nasty.
Anyway I'm getting sidetracked, with Fireworks I could also use it to make vector graphics. It was like a beautiful Swiss Army knife application to me; sure a proper steak knife and cork opener would work better than the miniaturized versions in an army knife but the army knife was just so gosh darn practical. Did I stretch than analogy too far?
Using The Gimp
When I graduated from high school and was no longer eligible for student pricing (at least I thought so at the time) I balked at the price and decided to force myself to use The Gimp. I had also just got one of the first Intel MacBook Pros (that I'm still using!) and it seemed a Universal version of Fireworks was a long way away. By comparison, third party developers had tirelessly ported the Gimp over to Intel within weeks, and the performance was so much better than Fireworks in Rosetta it wasn't funny.
It took a lot of getting used to, but now I have a photo and graphics editor that I can afford, with dialog boxes that have a sense of humour to boot :).
Personally the feature I found most confusing initially was the way the software dealt with layers. Particularly coming from a Fireworks background which lets you have multiple free floating elements in one space, The Gimp uses a separate layer for everything unless you specifically flatten it. Even if you copy a part of a layer and paste it, it becomes a new layer. Once you get your head around that everything becomes gloriously simple, but that mental roadblock was a tough one to get over!
When used with the really elegant and fun Inkscape software (which is also free and open source) you can do some amazing things. For example all the graphics on this site were made in Inkscape then imported into The Gimp for processing. Hardly a glowing endorsement because my tired old logo I originally designed when I was a kid is really quite terrible, but there you have it!
If you haven't used it in a long time, I encourage you to check it out again. The unified menubar and the inclusion of UFRaw in 2.6 has made it much easier to use.
The next step for me will be to look at it's scripting capabilities which are supposed to be insanely powerful.