PaintTool SAI

Pros: PaintTool SAI is legendary for how fast its brush engine is. Many brushes, you can blend better. The layout makes this software very easy to use.

Cons: Lack of guides and rulers. It's just not available for Mac which makes it very unfair for those Mac users to pay.

Overall: It wasn't until I used Paint Tool SAI, that I had fun learning to use my XP-Pen Deco 01 V2 graphic tablet to draw. That's because the program is compact, with just enough features that are easy to learn and use.
So, you'll find things like line-art layers with simple vector tools, masks, clipping and layer groups, but you won't find guides (except when transforming a selection) or rulers. But not everyone needs extra design features like that, just to draw what they want.
PaintTool SAI is a good application for a notebook or laptop, where you just want to draw something on the spot, and save it as a PSD file to import to other programs later. For what it is, SAI is a digital painting program, but if you want extra features like guides, animation, or even filters, this is not for you.

Pros: I love painting in this. Pair it with a decent tablet and you'll be making nice pieces easily. It doesn't have all the extras some other art software might, because it focuses primarily on the digital painting aspect. It's minimal, in a good way. It's more popular in Japan than in the west, but there is a small community of artists in the west who love this program too. I think lines in this look really smooth and it does a better job at mimicking how physical paintings actually look than some of the bigger name products out there.

Cons: It has far less features than what Adobe has got going on, and it's not an industry standard at all. Most people who use this do so as hobbyists thanks to Adobe. This also means you'll find less resources available as well.

Overall: For digital drawing/painting, I actually prefer this and many other lesser known art products out there over what Adobe has to offer. It costs less, and you can find the older versions for free which still work great. I prefer how pieces come out looking using this, and honestly wish Adobe didn't have so much of this market cornered so that more art programs could really flourish. It doesn't have all the bells and whistles that Adobe has to offer, but for digital painting, you honestly don't need most of that stuff anyway.