I've used a few different iterations of the GIMP on OS X, and although I haven't explored Seashore too thoroughly, it is a worthy OS X translation. While I'm not sure if this is really meant to be a complete Cocoa port, the UI's similarity to the GIMP - with the added Mac-friendliness – makes me optimistic. Overall, it seemed very responsive.

One suggestion/feature request would be to add live-updating previews for filters like Brightness/Contrast. It's clumsy the way it is now.