Pixelmator 1.4 adds new paint engine
With Pixelmator's existing tablet support, and the new brush customization features, you should be able to use Pixelmator for just about any drawing task. It's compatible with practically every image format, and has lots of intuitive little shortcuts that make it behave the way you'd expect from a good Mac app. The main gripe I've seen from commenters on the Pixelmator blog is the lack of a Photoshop-like "save for web" feature. Word on the street is that it will implemented in the next version, but that one feature should hardly deter you from checking out this great app.