Vs. the 800 lb Gorilla.

Acorn looks like an awesome little image editor, and it comes from Gus Mueller, the author of VoodooPad, so it's automatically worth downloading to check out.
Core Image effects, Python scripting, small footprint... looks like it leverages a lot of Mac goodness. I admire the move away from the paletted UI, but the combination of "tools" and "layers" into one palette might take a bit of getting used to. Many years of Photoshop make me want to break the tools off and put it on the far left, and put the rest of the palette on the far right.
But this is just the thing if you want to keep something around that starts up more quickly than Photoshop, especially if, like me, you're still working with a pre-Universal Binary version of Photoshop, which I never want to keep open for fear of Rosetta gobbling up all my RAM.