GIMP and products like it are written and maintained mostly by volunteers. There's also no guarantee about what skill set you get. If you go to GIMPs site, you'll see they are looking for a window's developer to help them fix bugs. That's a skill set they currently lack. Even if you have programmers, they will work on their pet projects and it can be hard to get people excited to work in the less glamorous areas like bug fixes.
There is also no big company to fund, drive, plan, set priorities, etc... GIMP doesn't have the huge budget like Adobe to get focus groups together to get feed back on their UI. They can't their whole team in a room for a week to story board it out. Considering the software is written by computer people, you'll also end up with user interfaces computer people like. These UIs tend to be less user friendly than the UIs that Adobe and Apple will put out aimed at the creative community.
So, what you get with GIMP is what you often get with this sort of software. A product that recreates the core functionality of the professional software, but, does it in a way that's less polished and less refined. They often emulate rather than innovate so they always seem to be some what out of date. If you're a full time pro, it's often in your best interest to shell out the money for the pro product. The extra level of features and refinement will help you do your job better, and will justify the cost. If you're a hobbyist, it might be a good solution. On the other hand, if you plan to buy Photoshop in a year or two, you'll be stuck with two separate learning curves.