Not sure if you mean painting or texture painting, but anyway, I actually painted for two years with a mouse, before switching to a tablet. Its valid for painting, but not for lineart based things. Graig Mullins used a mouse for 10 years, and while using a mouse, he was still one of the best in the field. Tablet is better, yes, but you can use mouse to paint and get good results too.
Pic related, Mullins work for Halo, painted with a mouse.