I tried to use everything as possible. From how GIMP (GIMPshop) that can't run well in Microsoft's ( but actually not really good too at LINUX, long respond FTW), and how fast Photoshop at Mac.
Playing with Photoshop script too(the javascript one), but it's not really that worth.
Also tried to do things with Adobe Flash (illustrations, interactive, and games), you can't imagine how annoying the brush if you never use it in drawing "a serious one".
Having Sai for a while, but its like "someone else doing my drawing" because the brush processor. I admit it have a good at viewing method (rotatable view) and great brush control. But still, i choose the expensive Photoshop because the habit (or might be because i started as a vector artist).
Manga studio is good too, have a brush correction for those who doesn't feel well with their Tablet. But still, it's just good at B&W, so might just worth for line art things and then export to Photoshop File.
Very portable Open Canvas? It's just if i need to draw something urgently with mouse at my friend's comp. Or... i might just use oekaki.
Ah... Corel Painter, i need to try that one.
Last try is Processing.org, it's good, since i love Science Art...
But WOOPS, is that an Image editor??
Well, it's just my opinion, so if it's not like that for you, just say so.
I've used Paint Shop Pro, Photoshop and GIMP. And I find all three of those so virtually identical that I don't even care. I use GIMP right now because it came installed on my computer when I switched to Ubuntu.