I've only used it for a short while so far, but the layout and tools are reminiscent of PS, while the drawing itself is pretty much what it would be in any art program. I've noticed CSP only has the box select and lasso select options, and doesn't have anything like the Select/Deselect brushes in SAI, which I miss. The stabilizer is also something you add to each individual brush instead of the overall project you're working on. The zoom is reallyyyy slow, even with a touch ring, but it still appears to have a lot of the same functions as SAI, but with SO MUCH customization available for brushes and pen sensitivity, etc. I'm still fiddling around with the sensitivity settings to get it comfortable for me without having to wear down my nibs just to get a decent weight line. CSP also has A LOT more built-in brushes for different kinds of pencils, pens, markers, paints, etc, as well as some pretty cool pattern ones. Other than that, from what I've noticed so far, it's really just a lot of the same tools and windows/palettes from SAI, but kind of repackaged so it just feels different.
I can't really say much about how it relates/differs to PS, though, as I only use it for some final editing in some cases.

Edit: Turns out CSP does have a Select/Deselect option just like SAI's Select/Deselect brushes.
I've also noticed that the cursor moves incredibly fast when I use the mouse with CSP open, regardless of mouse/cursor speed settings? :s
The stabilizer is rather iffy and only seems to work when it wants to, I think. Also, I'm only getting really faint, grey lines rather than thin black ones unless I'm pressing the shit out of my pen on my tablet and adjusting the pressure sensitivity settings in CSP doesn't seem to be making much of a difference to anything rather than the overall thickness of the line and how much it comes to a point when you stop drawing.