The GIMP is a powerful image editor/illustration package which can compete with many professional applications such as Photoshop Elements. However, it is somewhat lacking in terms of its user interface.
Launch the application and you are presented with four different floating windows: Tools; Tool Options; Brushes; and Layers, Channels and Paths. But for each image you wish to create or edit you need to open another window, and none of them are connected.
This means you can quickly end up with a screen full of windows, and confusion over what is going on could soon set in. The GIMP could certainly learn a lot from studying other image editors.
On the positive side, the software has all the features you would expect, along with some rather unusual brushes. It can even make use of Photoshop plugins.
It supports more file formats than any other image editor out there, but it does have a fairly high learning curve.