As far as scribbling and painting is concerned, this is an OK tool. However, I'm way frustrated at the lack of some bog-simple features. I drew some random squiggle and then decided I wanted it to be a different color. I selected it and ... you can't change the color! As previously reported in the MacUpdate reviews, there is no way to draw any kind of rectangle. There is a way to draw ovals, but it's pretty obtuse and no other kind of shape at all is supported. I also had the desire to drop down some text on my freehand technical drawing sketch but there is no way to enter text! I get that it'll be converted to pixels and not editable afterwards, but still... I suppose this is good provided you are ONLY sketching and don't ever want to change the color (or line thickness etc) after you've lifted the pen and seen what you've got. If you're expecting any kind of conventional drawing package features such as shapes etc. then you're out of luck.
I also wasn't that impressed with it's 'lagoon' toolbar. I thought it took up a lot of real estate for what it was. I called Autodesk's customer support with a couple questions and they just told me to go to the forums. So I went to the SketchBook forums to pose my questions. Aside from the administrator's first post in June 2010 (it's the end of August 2010 now) there wasn't a single post. Useless.
I know there are SketchBook fans here and they'll probably bag this review, but I'm sticking with my opinion...