To understand the effectiveness of banzai charges, you first have to understand that the tactic first really started during the Russo-Japanese War, where you get passages like this:

The whole Japanese line is now lit up with the glitter of steel flashing from the scabbard...Once again the officers quit shelter with ringing shouts of “Banzai!” wildly echoed by all the rank and file. Slowly, but not to be denied, they make headway, in spite of the barbed wire, mines and pitfalls, and the merciless hail of bullets. Whole units are destroyed – others take their places; the advancing wave pauses for a moment, but sweeps ever onward. Already they are within a few yards of the trenches. Then, on the Russian side, the long grey line of Siberian Fusiliers forms up in turn, and delivers one last volley before scurrying down the far side of the hill at the double.

That's from General François de Négrier, Lessons from the Russo-Japanese War, 69.

The "success" of the Japanese bayonet charges during the Russo-Japanese war emboldened European armies in the continued use of the bayonet during World War I and World War II - but the human cost to the Japanese was tremendous. A charge without supporting fire across open ground, often directly into the zone of fire of rifles, machine guns, and artillery. Sometimes, such as at the Battle of Saipan, the banzai charge (or repeated charges) made it up to the opposing lines and broke through, as given in D-Day in the Pacific: The Battle of Saipan, but the Japanese lost far more men than the Americans in the process. While I'm not able to put hard numbers to it, the general consensus is that the number of "successful" charges during World War II - those that overwhelmed the enemy position - were few.

Make no mistake, these were suicidal, last-ditch attacks. There was certainly a psychological advantage to a bayonet charge, much as could be seen in the American Civil War, which might have accounted for the success in the Russo-Japanese War - but the ever-improving firepower present on the battlefield made them a losing proposition.