I much prefer Inkscape over Illustrator for my vector art. I much prefer Audacity over... actually I'm not sure what's considered the direct commercial competitor here for post-processing audio. I mildly prefer Photoshop over Gimp for graphics design and tablet drawing, but after almost a decade of using each, they're practically interchangeable for me. I mildly prefer Blender over 3DS Max for making game models (though I admit it has been a while for me—I hear the recent Blender releases have amazing improvements). And that's not even taking into account the difference in license costs.
I'm not claiming to be an expert in any of these industries, but I don't think I'm completely naive here either. I can imagine there is a strong majority of experts who would never considere free/open source alternatives to things they've been using since they graduated their respective art school and paid thousands of dollars in licensing fees. I feel they're missing out, even though I'm biased towards open source.
P.S. I said "some of the best."