If watching a jury disregard mountains of damning evidence in favor of a charismatic gangster who calls himself a "gagster" seems a morally challenging proposition, well that's all part of the bargain in Lumet's topical drama.
We could have done just as well to watch the proceedings on Court TV -- free, as it were, to flip channels or get up for a snack and skip the parts that disinterest us, rather than be held hostage by 125 minutes of drive-by clichés.
It's hard to do anything in court that hasn't been done before. It's a static situation, and points are scored in tiny increments. No big witness-stand breakdowns, no tearful confessions. Thus, boredom creeps in.