I used Gimp for some time before taking the plunge in Photoshop.
The reason was that I couldn't get nice gradients with airbrushing in Gimp on my cintiq tablet, there were always visible steps in the gradient. I had to use quite a lot of dither to get rid of it. It could have been a problem with system performance of course, but I highly doubt that (6core CPU, Highend GPU, 24G RAM).
Photoshop gave me clean nice gradients on airbrushing without further tweaking. That was the original reason to take the plunge.
Of course there are other reasons to switch... plugins for example. There are tousands of good plugins for PS... some of them not cheap, but sometimes quite helpful. Like DDO / NDO for Texture painting.
In the meantime I found further art applications for specialized use. For drawing I started to use Manga Studio / Clip Studio Paint. Its awesome because of the Brush stabilization options. Makes it much easier to get clean strokes on crappy machines or with crappy digitizers. And all the options specialized to comic drawing are also quite nice, if you can use them. Not bad for 50$
Sketchbook Pro is actually another awesome drawing program for a cheap price... I instantly liked it more for drawing than both PS or Gimp. Must be the brushing engine combineed with the simplicity of the UI.
I still use Gimp regularly though. In some aspects it is much better than PS. For example, you often have to swap a color to transparency (For example for AO Maps). In PS you have to work with Masks.... easy enough, but thats like 3-5 klicks. In Gimp there is a simple function to swap a color for transparency!