No, its not too difficult, and in my opinion is better than PhotoShop.
First off, Paint Tool SAI is on par with OpenCanvas (another amazing digital painting program) in terms of size and ease of use. Significantly smaller than our mainstream digital media program, Adobe Photoshop, SAI is only around 6 MB, enabling those less-endowed computer junkies to draw digitally without lagging the hell out of their computers. When I used to use Photoshop, any picture over the size of 5 x 6 in would render my brush strokes into choppy segments of fail, which made drawing on Photoshop not unlike watching a stop motion sequence of, well, me drawing. Which made me very sad. On the other hand, SAI has proven its capacity of handling A4 size pictures without missing a beat. Of course, SAI doesn’t boast the digital enhancement tools found in Photoshop, but as it is aimed mainly at artists rather than photo manipulators and photographer, the tools and brushes it offers surpasses any other program of similar size.
Speaking of brushes, the sensitivity and customizability offered by SAI is pretty much unsurpassed. Their brushes have amazing sensitivity and would probably make any tablet user very very happy for at least a few hours (until they realize they actually have to sit their butt down and draw something for that to actually be significant. Yes, I’m talking from personal experience). These brushes also let users incorporate textures into their strokes while at the same time simulating the feel of natural brushes. For example, when red and blue and painted near each other, the border automatically becomes a mix of varying shades of purple, an effect that is still yet to be seen in Photoshop.