I have heard some positive things about GraphicsMagick when compared to ImageMagick. It seems to have better performance than ImageMagick and potentially fewer resources. ImageMagick has a habit of choking down at times on my vps for certain actions.
I believe flickr moved to using GraphicsMagic for their image processing a while back, so that speaks well for it. I have not used it personally, but perhaps someone here can vouch for its improvements over ImageMagic?
Not all hosters have ImageMagick library installed on their server (And even less have GM if you are not on VPS), most of the forums use GD and it's quite enough for forums. But I think for VPS users this will be a good alternative to IM.
I've had a couple of shared hosts that would install things if you just sent in a ticket. I guess it just depends on who you are signed up on. But yeah, I'd imagine this would be more appealing to those people who have more control over their environment.
Another thing to note is that so far, what I have read seems to indicate that most IM commands will work in GM. So based on that it might be easier than expected to specify that you wish to use GM, if its just a matter of pointing to a file and keeping the original IM commands.