The UI is very simple. Very deceivingly, so, in fact! there are a lot of simple tools it lacks that a lot of people wish it had, but it can be a good beginner's program. There's the color wheel, the brushes panel (which is a little weird to use compared to other things; you have to build literally all of your brushes by hand going off of what I call "recipes" by people). Under the brushes panel you have the textures it calls to for making an brush, and under that are the parameters for that brush. There is no reference image panel (as far as I've been able to find), which is unfortunate, so you'll have to either paste the picture into your canvas on a different layer, or use an program called Pureref. I like going for the "paste the picture itself" so I can color pick more properly.
- Very simple; a good beginner's tool, or for people who prefer simpler interfaces
- Brushes feel very unique to it, suprisingly, despite many other things having brushes similiar, due to how SAI handles the brushes.
- Very easy to do an softly-textured picture in it
- Universal stabilizer
- Has many guides and resources!
- Binary tool is one of the easiest to use, across programs that I know of and have tried, for aliased looks
- Resources (brushes) can't be directly imported into it (without a lot of effort in most cases, anyway)
- Lacks a lot of features that you may want, like more layer effects, repeating brushes, or so-forth.
- If you forget to import or remake an old brush you made, especially records of it, good luck figuring out that brush again
- No text tool at all; no line tool (you have to use an weird keyboard shortcut to make an line or shape)
- paint bucket is customizable but can be very finnicky, too
- No reference image area, so it's hard to keep colors consistent and similiar
- Making transparent images is a little hard to figure out; you have to save it as an png first of all, and then choose the "32 bit (every pixel transparent)" option.
- Most "brushes" for sai are actually just recipes, not elemaps, textures, or so-forth; that's what people mean they say a brush for sai, they mean the recipe
Here's Where to buy it for 5400 JPY (Japanese Yen) including sales tax (USD price and elsewhere may vary), if you so wish. It's worth it, and with it you get a key-- a key that also lets you use SAI 2 (also from the same site, but that's for another time, since hte process to get it is very different, weirdly!).
There's lots of resources. However, most of them are recipes, so I'll link the sai recipes category instead, then to some elemaps, textures, and so-forth.