Your question has been thoroughly answered at this point. I'll underscore some of the previous comments as an update.

Photoshop has really improved its brush system, damn them, and so I often don't feel like I have to leave Photoshop as much as I used to go to Painter for the painterly effects.

For illustrations, graphic novels, storyboards I thought Painter was great for some of the spattering brushes. 

Image hose! 1001 uses.

The dynamics of following angle and your motion path were key attributes that PS didn't have back then. 

My work path has changed and I don't use those items as much anymore so Painter has fallen in the background. It's been a while since I put both to the test. Painter 10 is my current version.

I loved Painter and yet its PS type tools were clumsy. And yet, I also seem to remember one could import PS plug-ins.