I've been taking Cellcept for about three years (2g/d for one year and 1.5 g/d thereafter) without good results (and also without any side effects...).
That is true that Cellcept is a more modern immumosuppressant but there are several clinical studies which have shown the lack of succes for Cellcept even associated with Pred in the treatment of MG.
My neuro says that Imuran and Cellcept are in the same class of immunosuppressors and therefore, there should not be any difference between them in term of effectiveness although Cellcept is known for its minimal side effects.
Good luck!
Maurice.