The Soviets used them effectively against the Germans, Chinese used them effectively in the Korean War, and the Vietnamese (correct me if i'm wrong please!) used them effectively in the Korean war.

To a large extent this is a myth: human wave attacks as exists in popular imagination -didn't- work. The Soviet red army succeed against the Germans because they used a combined arms force of tanks, infantry, artillery and warplanes that eventually matched and outfought German combined arms. The Chinese during the Korean war succeeded in the first phase of their intervention because they achieved both tactical and strategic surprise and were able to turn tactical disadvantages of a light infantry force into an operation advantage by camouflaging them against UN reconnaissance. They were able to infiltrate corps size units into Korea without acknowledgement by MacArthur and were frequently able to infiltrate large units onto the flank of UN forces without detection. They used surprise, ambushes, flanking, night attacks and superior concentration of force against an opponent poorly prepared for such tactics to route a superior equipped force from North Korea in a matter of month. The image of Chinese or Soviet drowning their enemies in a sea of bodies is in part a creation of Hollywood and popular culture depictions of an Asian horde overwhelming western positions with superior numbers. It downplays the real sophistication of Soviet/Chinese armies and does them too little credit.

More can be written, but even just about World War I, you might like to see the "Trench warfare" section of the World War I section of the FAQ, by /u/NMW, /u/DuxBelisarius, /u/military_history, the now-deleted elos_, et al. A lot of people have the impression that almost all World War I was unchanging tactics with tons of people going over the top and being massacred, but even just on the Western Front, tactics evolved. A nice brief summary is "I kept hearing about significant infantry combat doctrine changes during the first world war. What were the main changes among western military from 1914 to 1918?" by elos_.

You might also like to listen to episodes 35, "WW1: Myths & Misconceptions", and 36, "WW1 After the Somme" of the AskHistorians podcast, also by elos_.

This is not to discourage discussion. More questions, data and debate are welcome.

Since Vietnam hasn't been mentioned very much this far in the replies I'll give that a shot, and it ties into the description for the use in Korea.

The Vietnamese systematically used ambush attacks, where they snuck up as close to the defenders as possible before commencing their attacks; initially with little to no artillery support, but it didn't take too long till more and more artillery and more and more machine guns were available, increasing the success rate - or at least the manpower v manpower ratio - between them and the American/South Vietnamese defenders. Their way of concentrating force was as we know usually by sheer manpower.

Even then such attacks often - but definitely not always - supposedly had horrendous casualities (It was often hard to find/recover enemy bodies in any significant number, leaving a large room for speculation about their casualities in general during the war). When you are firing at point blank ranges and one side is in camouflage uniforms lying on the ground in cover/concealment while the other side - depending on the type of North Vietnamese unit - may wear pretty much anything and has to get up on their feet to make the final dash onto and through the enemy defense, then the defender has a definite advantage man per man as long as they can cover all directions they are being attacked from.

These attacks were generally also performed against positions where there was no real defense in depth. Against more well prepared defenses they performed more well coordinated attacks and usually with MUCH more artillery support, and later in the war even with tanks such as Soviet T-54s. When there are no further lines of defense to hold off against, and when there is no immediate counter-attack coming from terrain you don't control (in Vietnam counter-attacks - when at all available - often consisted of airborne infantry flown in by helicopter, and often the control of landing zones was horrible, or the enemy was already gone by the time the reinforcements arrived), it does not matter much if you as the "victorious" force is too weakened to hold the ground you just took. They could simply leave it and vanish into the jungle the moment they felt like it.

I.e. the nature of the war itself in Vietnam increased the viability of human-wave attacks because you had no need for a power reserve to hold the ground you just took, unlike in conventional warfare. After all the ground war was mostly fought on South Vietnamese soil, and the North Vietnamese had no need for holding that ground, and their own territory was at worst being bombed, and for a fair part of the war not even that since it was considered a No Fly Zone. The ability of the opponent (USA/South Vietnam) to reinforce and counter-attack was limited, and it was difficult for close air support to attack with any accuracy against targets they can't even see; they generally had to indescriminitaly bomb jungle just around the defenders and hope that they actually hit something. Without being under threat from other forces than their intended target as well as having the initiative for both when and where to attack it eas easier for the North Vietnamese to conduct safe operations.

As a last point I definitely want to reinforce the emphasis on that the norm was far from attacking with a large wave over open ground: Although human-wave attacks routinely were employed they were employed under as favourable conditions as humanly possible, against an opponent with poor ability to react to such attacks, and at a time and a place chosen by the attacker the vast majority of the time. In almost every single engagement the North Vietnamese fired first, i.e. they had the initative, and that is critical to success: If they are forced onto the defensive and have to counter-attack with human-wave attacks to put enough pressure on the chosen point of counter-attack the defending Americans/South Vietnamese will have a far greater ability to defend since the element of surprise is gone, or at least reduced.

PS. One strength that the Americans and South Vietnamese did have was quick and effective artillery. Being able to fly artillery into new Areas of Operations and quickly set up firing bases, as well as using forward observers in the front-line units, meant that fire support often was surprisingly quick and accurate given the terrible topography of the country (hard to have good maps, to know exactly where you are, exact altitude above sea level, etc). However counterbattery fire was difficult and lacking, and the North Vietnamese often denied the use of effective artillery support by performing those very close-in attacks.