GraphicsMagick fixed all the memory and CPU problems that ImageMagick was giving us in a large wiki farm production environment. That alone is a reason to switch to it.
I switched to GraphicsMagick a few years ago too. I find that its command line options make more sense and that it produces better results. I do ad-hoc tests from time to time and and the difference between the two isn't earth shattering, but I get the feeling that GraphicsMagick is a better project for some reason (maybe it was something I read once).
do you have a covering blog post about that?
Unfortunately not. That occurred back under different management when talking about internal infrastructure changes was discouraged.