I was really tempted to buy this program. I bought ArtRage instead, for the following reasons.
1. Most important to me, Sketchbook Pro uses a network-based phone home system to register software keys. That means you need Autodesk's permission to install the software; your computer contacts their servers over the network and ties the key to your specific computer so you can't install it anywhere else. If you change computers a lot as I do, that simply won't work. Not only that, it invades my privacy and it's just rude to treat paying customers like thieves.
Let's not forget that pirates will not have to worry about this copy-protection. They have cracked it. It overwhelmingly penalizes and alienates paying, good customers, with no valuable return to Autodesk.
2. I didn't see a way to draw simple shapes like a square or an oval. What's up with that? I would expect a sketch program to support basic shapes and stencils. ArtRage does. (Feel free to correct me if I am wrong on this point, I didn't do an exhaustive survey, but with some effort, I couldn't find simple shapes.) Also, where is the text tool? I'm sure there must be one somewhere and I'm just missing it, but obviously my efforts could not reveal that feature either.
3. The program costs four times more than ArtRage.
Having said all that, the program per se is not bad. It's got a good selection of brushes and drawing tools. I'd have to say as a program per se it is flawed, but quite good nevertheless. But given its offensive copy protection, its lack of some basic features, and its much higher cost compared to the competition, I simply cannot recommend it.
By the way, if your needs are very simple, you might try Scribble. If you need real power, that app is not for you, but if you just want to do some simple drawings, it might be great.