Not very.
Shield walls were much slower events, with a focus on staying in formation. You can't do that while charging. This video shows a Saxon shield wall, which honestly, isn't any different from a Viking shield wall. http://www.youtube.com/watch?v=33BWMw07YiA
The one on one combat techniques are also not well represented in the show. See this video here: http://www.youtube.com/watch?v=dkhpqAGdZPc.
Edit: in the second video, the good part is when he uses the sword and shield together and starts putting all the concepts together so you can see how they fought.
Two very important aspects of warfare among Vikings and Saxons was thrown weapon skirmishing and shieldwalls.
The most common weapons were spears (light for throwing, heavy for thrusting) and one handed axes. They also had a short knife/sword known as a seax. Wealthier vikings could have a sword that was longer than the seax. Spears are cheap, axes were practical. There were larger two handed axes used by huscarls, but there weren't as many of them; though more appeared towards the end of the Viking Age.
Their defensive armament would be a large wooden shield (could also be used to bash), thick cloth tunics, various leather armors (usually a helm), and for a richer Norseman/Saxon a chainmail hauberk.
Most of the participants were levies who were farmers, craftsman and the like; and that is accurately portrayed. They fought only when needed and had other lives. In a recent episode, Ragnar even states he is a farmer, not a warrior. Only a few would be professional warriors, if they could be called that. They were Huscarls, who were more of bodyguard/retainer. There may have been the debated one or two berserkers,too. In the show an episode looks to have Rollo acting like a berserker, attacking wildly with a large axe and no armor.
They had a prolonged period of skirmishing with thrown weapons, mostly javelins and spears while they remained in their shield wall, or would run forward briefly for more projectile energy. Younger and braver warriors would run out and grab grounded javelins and toss them back at the enemy. They portray some throwing weapons, but not nearly enough and it occurs incorrectly during the melee phase. What is correct is that neither used many archers, but a few would be present.
The melee phase would be where the shieldwall collided, which would be after enough missiles were thrown. They tried to remain in their shieldwall as best they could because their primary armor was their shield and the shields of the men next to them. I cannot confirm if they would advance at a slow walk or if they accelerated, but it seems likely they advanced slower in order to maintain cohesion. Using thrusting spears they would try to find a gap between shields. With an axe, they would try to "hook" the shield and pull it away so a fellow Viking could make the kill, which is portrayed in the show. Often they would pull back to regain stamina and resume skirmishing, only to engage in the melee again. The battle was over when the losing side either did not re-engage their opponents for another melee (common), or if the losing side broke and fled to be slaughtered (rarer). The battles were not often high casualty affairs due to the nature of shield wall and the repetitive withdrawals.
Another accurate portrayals is that horses were not commonly used nor was a large amount of armor. At least they do not have horned helms in the show.
Sources: Scattered amongst vocab
http://www.regia.org/listings.htm
http://www.millennia.f2s.com/nature.htm
Could you describe the combat? Techniques used, or a video? I haven't seen the show.
I'd like to add a few questions or specifications. Were shield maids a thing and was it common. I heard somewhere that Vikings didn't use bows at war because it wasn't honourable and instead,they threw small axes, the reason for that being a) a shield wall could stop the arrows and b) you could throw the axes over the shield. Is there any truth in it?
along the same lines, the History show Vikings has the Vikign soldiers equiped with mostly leather armor, axes, and wooden shields, while the saxons all have chain mail, iron reinforced shields, and swords.
Would the Vikings not claim the equipment of the dead and use it? It seems they would value the chainmail and swords.