GIMP is more of a Photoshop substitute (Raster image). Inkscape is what you want for an Illustrator substitute (Vector Image). Both programs are good, I personally find them annoying, but I am able to complete most of what I would do in Adobe in GIMP or Inkscape. Most of my grievance is that I have been using adobe products since the early 90's, and simply cannot adapt.
Many people prefer GIMP and Inkscape. There is actually a interesting video where vector image manipulation is done side by side in both Illustrator and Inkscape, and continually Inkscape proved to be the faster tool. I am sure the video is biased to a degree though, as I noticed a couple shortcuts in Illustrator that were not taken.