I also have Autodesk Sketchbook Pro, which is a wonderful program and it is criminal how little I use it. Those three are my trinity of killer drawing programs. I'll also use Photoshop for various things. My current set-up involves sketching on Manga Studio Pro - then proceeding to using MSP brushes for more expressive line work, or transferring the sketch into Illustrator for more precise inking. Obviously no program can do this, but it is interesting to see how different painting programs are to one another. There's always the quest to find the perfect set-up with that magic tool that will transform my lousy sketches into something magical. I can't help looking at what other painting apps have to offer.
Krita is a full function digital painting program with a loyal fanbase and more importantly, a price tag that can't be 's free.