Gimpshop is OK - it mostly works, but there are still significant differences between it and Photoshop. Seashore, in my opinion, is not worth the trouble, since it's much too limited in its functionality (or was the last time I tried, which was a month or two ago).
If you requirements are too high, Photoshop Elements 3 for the Mac is probably the nicest image editing app available for the average user.
HTH