Leonardo is a great application that is currently only available in it's BETA version. The cost for the software is only $39. until it finally launches and then the cost will be $79. However in the BETA version you would be allowed to continually upgrade all the new features and improvements as they come out.
I really enjoy sketching in this application because of the natural look of the pencil tool as well as the what in my opinion is the best ink brush outside of Clip Studio. The above sketch was done using the pencil brush. Below is a video showing how I sketched the drawing pictured above in Leonardo.
If you are interested in purchasing the program it is available here.