Plea bargaining is efficient in that it does reduce costs. However, it can be seen as unjust. It allows people who are guilty of one crime to get a reduced sentence by plea bargaining to a lesser crime. It can induce people who might be found innocent to plead guilty. Either way, it is arguably unjust and we are giving up some degree of justice in exchange for lower costs.
I think that the plea bargain process is more effective in cases where it is obvious someone is guilty, and you can avoid the cost of a trial. Trials are expensive and time consuming. The plea bargain process is not effective when it is used to browbeat a suspect into confessing.
Hm. I know nothing about "plea bargaining," but to answer the question, I'd have to know what "effective and efficient" means. I can think of effective as meaning "ensuring a prosecution, even if for a lesser offense and punishment." In this sense, it is "effective" because it ensures an outcome that would not be ensured were a case to go to trial. If "effective..." means "likely to ensure the guilty are punished," then plea bargaining is "inefficient" because people might agree to lesser offenses/punishments simply because they are afraid or because they fear a greater offense/punishment though they are actually innocent. There are probably other definitions and consequences for them. In all, it really depends on what you mean.