Looking at the roadmap: http://wiki.gimp.org/index.php/Roadmap it seems there won't be non destructive editing for quite some time either (several versions).
I've tried Gimp numerous times, and the lack of smart objects/filters makes it a complete non starter. Generally I found I could do most of the things I wanted to in Gimp, but they required many, many more steps.
Though it won't be several versions; they're looking to move to a much quicker release cycle - so shouldn't be as long as it took for GEGL to get included in the first place.
I've tried Gimp numerous times, and the lack of smart objects/filters makes it a complete non starter. Generally I found I could do most of the things I wanted to in Gimp, but they required many, many more steps.