This depends on how you define success as director of the FBI. Hoover was notably successful in some ways but could be seen as a failure in others.
Hoover was quite successful in making the FBI a powerful and fairly effective crime fighting agency. He promoted, for example, the use of science and technology for fighting crime. He also used publicity very deftly to improve the image of the FBI and to increase public trust in the bureau.
However, Hoover can be seen as a failure for his actions that seem to have violated the rights of various Americans. Hoover had no qualms about illegally wiretapping and otherwise surveilling many innocent people because of their political views. His keeping of files on such people constituted a major invasion of privacy. To the extent that the FBI is supposed to be a protector and not an invader of personal rights, Hoover can be seen as a failure.