SAI is amazing. If you want a painting program that paints better than Adobe products and is cheaper than Adobe products and is more beginner friendly than Adobe products (am I repeating myself here? lol), then SAI is right up your alley. It's easy to learn, you can make your own brushes, it has great pressure sensitivity options and stabilizer tools, its blending tools are a lot more exact and cleaner than Photoshop (which is why I say it "paints better"; Photoshop just shits all over your work when you try to use those damn Smudge tools :I) lots of customization for palettes and swatches, and very basic, easy to use tools to adjust hues and saturation levels, without it being complicated like in Photoshop (although it doesn't have as much as Photoshop when it comes to adjustable color levels and stuff). It also comes with the ability to make linework layers, which is built on vector lines, so if you like using vector tools, it has that too (not as much as Illustrator, but enough to get the job done). And of course, you can save PSD files in there so if you want to cross your files with Photoshop, you can. And fully customizable keyboard shortcuts.
It doesn't have text tools or shape creators though. So I'd suggest getting Photoshop if you don't have it already or something that can let you make text and shapes and stuff that SAI can't do. SAI is purely for drawing and painting, and it does it very, very well. Definitely consider getting it (especially if you've been saving up your money for it anyway; don't be afraid to give the free trial a go either, so you can get some hands-on experience with it before you spend money on it).
On its own, SAI is user-friendly and purely meant for drawing and painting. So if you have Photoshop, I'd highly suggest pairing it up with SAI for a perfect comic-making package ^__^