Was J Edgar Hoover considered a success or failure during his time as director of the FBI?

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.