The book Einstein's Lost Key by German physicist Alexander Unzicker is all about that idea and its history. Unzicker argues that it's still a good idea, hasn't been falsified since it makes much the same predictions as GR, and we should look into it further.
I read about criticism of General Relativity, that I haven't verified so grain of salt please. The criticism is in that the equations are so complex that only the simplest limits of them are studied and so the theory is therefore not verified.
