I use SAI primarily. The problem is...the most recent "dl-able" version doesn't let you save unless you delete something from the directory every time. That's the biggest problem...and it is quite a problem, so you might want to just dish out the $50.
Nonetheless I prefer doing linework in SAI. It's very simple and user-friendly. Rotate canvas is a plus, although I believe the most recent version of PS has that as well. I also like how paints mix, although I'm sure PainterX beats it in that field. PainterX just has way more options for brushes.
I used to use OC, but I prefer the tools in SAI so I don't use OC much anymore. Version 1 has an interesting networking capability, although I'm not sure how useful that is in practical situations.
Most concept artist-level people use Photoshop and/or PainterX, but I think below that level SAI works very well. And I've seen some amazing work done in SAI. 4mb program ftw.