I used the Gimp for < spam > http://rommelhok.com/en < / spam > (48 pages worth of old fashioned adventure comics). Not for the drawing itself, but for the cleaning up, the coloring and the lettering. It was fine.
Everything worked without a hitch, once I got used to some new shortcuts (which can easily be set to the Photoshop equivalents if you want) and some minor differences from Photoshop, some of which were, in fact, improvements (the color selection tool is much nicer in Gimp than the color range selection box in Photoshop, for example).
The only area where Gimp definitely falls short is type setting. In Photoshop, you can tweak kerning, spacing and font size within a text layer for every character individually. In Gimp, you can only do that for the layer as a whole. This wasn't much of a problem for me, however, since computer lettering is already inferior to hand lettering (IMHO) and using Photoshop's superior tools wouldn't have decreased the suckiness of the lettering all that significantly.
The other main shortcoming of Gimp, of course, is that it can't do CMYK colors but that will only become a problem when someone decides to publish my comic, in which case having to redo the colors will be a nice problem to have. I made one printed copy of it for myself at the local copy shop using the RGB Gimp files and that turned out beautifully.