The mere fact that vast amounts of Gedolei Yisroel sit on the dais at the Siyum HaShas in New York, Eretz Yisroel and many many cities around the world. Including almost the entire slate of the Moetzes Gedolei HaTorah, both in the U.S. and in E.Y., speaks volumes about their support for the Daf Yomi system.
And to top that off, not that the above point even needs any additional support, the total lack (or close thereof) of any public dissenters, against the Daf Yomi system by any Rov of stature, speaks volumes too. Their silence is deafening.