It's not hard to create great artwork using Painting and a graphics tablet. Just enable the stuff in the Brush Settings dialog (you can select Brush Settings from the Edit menu (-B) or double-click the paint brush tool to get the dialog).
The extra options in the Brush Settings dialog for tablets
With the Size option checked, the harder you push the stylus the larger the brush stroke will be. The largest you can make it is set by the same old brush size slider.
If you want to vary the colour of your brush strokes, make sure Colour is checked. By default, if you press lightly with the stylus you'll paint the Fill colour blended with a lot of white. If you press harder with the stylus you'll get more of the Fill colour.
You have two more options when it comes to colour blending. By using the popup menu in the snapshot above, you can also choose to blend your colours with black or the Stroke colour. The Stroke colour is normally used to frame shapes and lines with colour, but this option lets you use it with your paint brush too.
Blending the paint brush with the stroke colour gives you a lot of flexibility. The lighter you push, the more of the Stroke colour you'll get. The harder you push, the more of the Fill colour you'll get. With practice, you can paint excellent shading and highlights. Whatever.
Tips on Changing Colours
You'll find yourself needing to change the Fill and Stroke colours a lot. Normally, setting one of these colours means making sure the correct icon on the colours palette is selected, and then picking the colour you want.
Colours palette set to choose Fill colours.
However, there's a quick shortcut to setting the "other" colour (if the Fill icon is selected, the "other" colour is Stroke). Just option-click a colour. You can even do this with the dropper tool.
To make your life even easier, be sure to read up on hot keys. And don't forget, it's often handy to make your own colour set if you find yourself using the same colours often.