So would GIMP be perhaps the best alternative then?
You can try Pixelmator free for 30 days to see if you like it. It uses a lot of realtime FX and tools that I assume are powered by Mac OS X's Quartz. Again, it is very reasonably priced ($29.99
). The creators have tried to concentrate on the most frequently used tools found in Photoshop.
Yes, GIMP is free and it has some very powerful tools, such as LAB color. You can play with that as long as you like, and there are many tutorials on-line, as well as numerous books.
I have both apps, including an older version of Photoshop, and Painter. There's never any "right" way or "best" way. It's art.