I'm a graphics artist and I've used both. I'll always take Photoshop hands-down. It seems to me that GIMP goes out of its way to NOT be like PS, down to using different quick key strokes to accomplish simple and repetitive tasks (zooming, tool usage, etc). When the brain is programmed for so long to operate a certain way (PS), acquiring speed and efficiency in that product, and then forced to operate another way with another product (unlearn what you have learned), it fails - epically. Additionally, going back to first product after re-training the brain then becomes problematic and all the things you knew without thinking you actually have to think about again - the muscle memory is gone - all that is left is infinite frustration. For you UNIX folks out there, it's like being in the "vi" camp and refusing to EVER use "emacs".
The one thing I do like about GIMP is its file-handling capabilities - it can naturally detect a file format at save-time without having to select it from a drop-down menu. It also automagically turns files you give a "GIF" extension into indexed coloring (you still have to choose the kind of palette, IIRC), but it does save some steps where PS requires more clicks.
That aside, I despise the way GIMP handles open windows and drawing tools. I find dealing with images with transparent backgrounds to also be quite clunky.
And I guess that's the core word I would apply to GIMP - "clunky". Yes, it's great for those who can't afford PS, but if the developers focused more on ease-of-use and intuitive interface design, it could definitely be a contender for the freeware graphics crown and maybe - just maybe - start chipping away at PS's ivory tower. At this point in time, though, not a chance. I have yet to really find any freeware app that comes close to PhotoShop with regard to usability.
In summary, yes, most of the same functions are there, it's just really hard to get anything done, IMHO, and I would think even someone who has never used PS before will also find it cumbersome.