This is indeed an age-old question often asked by attorneys in criminal cases.  The answer to the question often depends on a variety of factors.  Issues such as the defendants background, character and credibility are among the top considerations in any case.  Most criminal defendants do not take the stand in trials.  Some pundits allege that the jury needs to hear from the defendant and have them say they are innocent,but this thought process is rather simplistic and fails to take into account the respective burdens placed on the prosecution and the defense.  The Constitution gives all defendants the absolute right to not testify and places the burden of proving every element of the crime directly on the prosecutor.  The bottom line is the ultimate decision to testify is very fact specific and always rests with the defendant himself.