Ranked in these QuestionsQuestion Ranking
Pro Free and open source
Krita is completely free and open source. They have raised a couple of successful Kickstarters in the past to get the initial financing and now they are accepting donations.
Pro Constantly updated
Krita is getting constant releases with updates and bug fixes. New features are added at a pace that far surpasses the other alternatives.
Pro Easy to use layout
All the most used and useful tools are easy to find in Krita's UI and are often just one click away. They are not hidden behind menus or dropdowns.
Pro Very customizable
Although the UI is rather busy, Krita is very customizable. The editing window can be themed and the sidebar can be customized extensively throw many dockers or panes.
Pro Easy editing in a tiled view
Tiled view that shows your image tiled in the editor, and permits you to edit it as you are seeing it tiled. If the brush passes out of your texture, it will just automatically wrap back the painting to the other side of the original texture, while permitting you to paint and see the results on any of the tiled "clones". (the shortcut key to activate this is w by default)
Pro Amazing support for displaying brushes
Krita's preset brushes are one of the default dockers. Each brush has a preview on mouseover that shows a detailed view of the type of brush involved. All the brushes also have useful and descriptive names such as "HP Pencil" or "Textured Fuzzy".
Con Documentation is lacking
Krita's official documentation is incomplete in some areas, especially for new features that are constantly added. But this is compensated with it's great design and usability which makes it easier to understand how things work.
Con New features are not tested thoroughly
Since releases are so often and with little time between them, usually new features are not tested a lot and this can bring a lot of bugs with them. Which fortunately are quickly patched in the next release.