I'll second that. I did this run from Lithgow through to Kurrajong last week in a hire Corolla. Mostly a bit too fast to be really fun and legal in an MX5 but the scenery was just outstanding - every bit as good as the Great Ocean Road!
I've done a few runs up the Bells Line, including one with our club. Makes for an excellent day out: meet at twin servo's, up Bells Line, stop at Mt Wilson for lunch, then stop at Leura for afternoon tea, then battle the traffic home. I have 'pace notes' if anyone needs them (showing stops, odo readings)