Quite honestly, I've learned to like Inkscape as much as if not more than illustrator... That being said I would most likely still use Illustrator if I had a copy of it.. but inkscape does almost everything, and yet to me it seems to do it in a more straightforward way (as far as drawing anyways)... where it lacks I find is in the features, mainly that filters are a bit hard to figure out, using palettes is non-ideal, even things like the stroke, fill, and gradient UI leave some things to be desired... The only thing i've found that is truely not doable in Inkscape is gradient meshes, which of course are a very cool thing.

But that being said it is definitely worth using as a free alternative, and none of those issues really makes it impossible to use.. usually just takes a bit more time to get exactly what you want...

GIMP I've really just started using for something more than cropping and quick editing of pictures... to me it seems even less refined than Inkscape, but seems to be much more powerful... the scripting capabilities I've seen RobA come up with are amazing, and I'm sure it's just a matter of getting used to the software as well.. Again I'm sure there are a number of features that just aren't possible yet compared to PS, but again.. I'd say its probably the best free alternative out there...