Painter is still buggy in places. I've been using ArtRage from time to time. No way it's as comprehensive as Painter but the price is right and it's almost as frustrating as real oil paint.
Yeah blame MickeySoft for the RIFF ambiguity thing. It meant Raster Image File Format but when BillCo "copied" the Electronic Arts formats they used it for Resource Interchange File Format in Windoze Media stuff, and changed the endian-ness for the Wintel platform. When you work with their clunky AVIs (Audio Video Interlace) and such you run into that context of RIFF in a hurry.
They're all descended from IFF, another Electronic Arts Mac/Amiga 68xxx processor thing. I drive past their new building frequently and salute.