Sketchbook Pro Rules!
I've had sketchbook pro on my PC Tablet which I use to teach my Schoolism classes, but I've just recently bought a copy for my MAC/Cintiq . . . it's SO MUCH FUN! It's an absolute blast sketching and drawing with Sketchbook Pro, I highly recommend it to anyone that just loves drawing and sketching!!
I did these three shark sketches just for fun, messing around with the program . . . each sketch took about ten minutes!