A while ago I acquired Paintstorm and field tested it on one of my works that I was doing for art challenge at Cubebrush
. By the time I already had all sketches done and I used Paintstorm to paint/render final image. I very much enjoyed the brush engine! It allows for creation of rather intricate brushes with subtle control of many parameters, well structured and is not loaded with excessive and demanding features like "watercolor" emulation etc. I think it far exceeds the PS brush engine and can compete with Corel Painter's in many aspects. I also enjoyed many UI features like individual brush hotkeys, slider hotkeys, reference layer etc. Of course there are many things to improve, bugs to fix and features to add.
But considering a symbolic price I wouldn't want to push too much on that. While working in Paintstorm I was writing down a list of fixes that I think would benefit it and if development team is interested, I could share it. But again, overall impression is very very positive and I hope the software will develop further. Kudos to the Paintstorm team, well done!