Well, I'm happy to report that GDGraph is being used quite happily by the Webmaster of Map my Run, which for me is completely encouraging to continue moving on with this project. He's given us credit in the last question of this FAQ.
Who knows? Maybe it'll move on to second place above Libchart in Hotscripts where we stand on third place in the freeware graph PHP classes. First place is quite a long shot: JpGraph is very hard to beat right now, but I'll settle with second for now =).
EDIT: (2010-09-01) I don't know when this happened, but "Map my Run" isn't using GDGraph for PHP anymore. Pity.