I’ve been playing with this a little, and so far it seems like a very useful program – much more intuitive than GIMP, which is a real horror to use, in my opinion. Krita has some very nice brushes. I’ve only had a short time to play with it, but so far, it looks great – especially for students who want a quick photoshop alternative. Krita saves in .psd format also.
It’s available in 32 and 64 bit versions. To date, there is no mac version.