Spenta Wadia wrote a semi-technical review of string theory that might be useful for many TRF readers familiar with some related physics concepts but not with those of string theory:
But it also presents the correct microscopic calculation of black hole entropy as a nontrivial check that every theory of quantum gravity has to pass. In his opinion, holography might become the ultimate underlying principle that defines string theory at the end.
You know, the microscopic calculation of the entropy could have led to a wrong result: the correct result is thus a nontrivial piece of evidence supporting the theory. Because the result could have been wrong at the beginning, the theory is falsifiable. That's one of the important features that put string theory above the would-be competitors.
The hypothetical falsification would be based on observations only indirectly: the black hole entropy hasn't been directly observed but it can be calculated by semiclassical techniques involving the laws that have been established empirically. But this indirect character of the falsification doesn't matter: falsification based on indirectly known facts is as powerful a killer of wrong theories as falsification based on direct observations. So far, all other conceivable competitors have been killed in this way and chances are that all of the non-stringy ones will be killed in the future, too.