Seashore is no where near as complex as Adobe Photoshop, and it isn't entirely intuitive in its design either. It looks like many other image editors on the Mac, like Pixen, which is nice. Each tool bar is separate, and there are even layers you can work with, which is a huge plus.
Seashore's biggest problem is it is hard to figure out how to do what you want. Unless you have used GIMP, you may not be able to figure out how to even crop an image, something that should be simple. The steep learning curve is a real set back for the casual user.
The interface is similar to that of Adobe Photoshop, so if you have worked with that it might be simpler for you.
Seashore comes with a very good guide. It is not a complete guide, since the software is still being updated, but it will most likely serve your needs. Once you find out how to properly utilize it, you will love what you can do. Cropping images, burning and dodging, textures, paint, smudging and several kinds of cool effects are available.