EDIT: Just tested Gimp's Make Seamless - it is quite bad.

Yeah : i have also tested it some time ago, and it is not really good for all things.

Well, i'll have to see if i buy seamless texture generator, caus its seems complete and able to do perfect results.
Well i've tested the tool , and finally it depends on soruce texture : sometimes very good results, sometimes you can feel the tilling of texture.

I think finally , by hand is the best, caus with all tools like smudge,blur,copy/paste etc ... you can
relly make disappear the seems.