Here's something kinda interesting. I just attended a full day class with Fay Sirkis who is known for her photo paintings. The class covered some painting history and styles which was nice.
I thought the class was centered around painter... which most of it was... but there was some Photoshop stuff involved as well. She showed how to prepare our images in Photoshop before bringing into Painter.
So the main workflow she used was camera to
PS to Painter. Then she showed us how
PS could accomplish some of the same things as Painter and it really made me think.
Painter is a neat program but... if you are converting a Photo to painting... most of what can be done in Painter can also be accomplished in Photoshop. She even showed how to create the brushed stroke effects in Photoshop as used in Painter in case some of us lacked Painter software.
Not trying to persuade anyone from getting Painter... but if you already have Photoshop and considering Painter... I suggest giving Painter a test drive first. You may find that digging into the
PS brushes with a Wacom can accomplish many of the same effects... though if you really just want to smear oils and watercolors around and like the additional brush choices and behaviors... then Painter is worth a look.