I can't use the GIMP for a few reasons, one of which is my work flow is so hinged on Photoshop's way of doing things that I can't migrate. In terms of usability there are many areas where GIMP just can't compete in.
And herein lies the problem.
This really just the same old, "Well, I'm not used to using it and don't know how to all of its tools, therefore it's not as usable/efficient and thus not as good." That's nonsense.
Though I don't blame you. Part of the problem is a lack of knowledge. While there's certainly a wealth of information: FAQs, HOW-TOs, etc. on GIMP across the web, it isn't always the most accessible. Nor do they usually (if ever) go into full detail. There's also a severe lack of literature on the subject--go into a bookstore and be lucky to find a single book.
The thing of it is, though, since it's open source, with a little know-how, one can figure it to be as usable and efficient and he needs it to be. There are also several builds out there (like five or six) that have been designed to function more like Photoshop.
There's also the CLI functionality that, once a person learns how to use properly, will wonder how he ever got by without it.