I use Krita "full time" (as in, quite often, it's not my job) for painting. It really is amazing for digital painting (with a graphic tablet), I definitely prefer it to Photoshop (and others) for painting.
However in other areas it's a bit lacking (vector graphics, and anything beyond simple-ish image manipulation).
A huge thanks to David Revoy who releases his "source files" and brush kits, they are tremendously helpful in terms of studying techniques he uses, and the brushes are simply and increase productivity a lot.
(edit: formatting/typos)
It's very stable and very polished, and really intuitive to use. Out of all the open source graphics/image editing tools, it is definitely my favourite now. It rivals or even surpasses its proprietary competitors in both features _and_ usability, which is no small feat.