DDO after Turbine took over.
(Highly is relative, game went from heading to the trash bin to making immense profits and sustaining a rapid expansion with constant content updates).
http://massively.joystiq.com/2010/02...ragons-online/
1 million+ subscribers as of 2010, even more now.
Not quite pure F2P, more of a hybrid subscription model (you can chose a regular subscription or chose micro transaction system, subscription model gets all the goodies, micro transaction must use points that they either earn in-game or purchase in bundles).
DDO has become a benchmark for MMO success stories. From a failure after launch that including shutting down servers in the entire EU region to a resounding success that had servers bursting at the seams.
