It isn't often successful, except when defendants are genuinely ill - courts look at these claims very sceptically, even after expert assessment. And as the law now stands, someone who commits a serious crime while mentally ill is likely to be detained indefinitely i.e. effectively the same punishment as if he was a 'sane' murderer.It is an annoyance of mine when people use mental issues in an attempt to gain the sympathy of the courts when they have no right to do so.