[@JaceBeleren] they were definitely used historically. Their primary use was in formation with pike and against pike. I'm sure you have heard of 'landsknecht' or 'landsknechten', who used this type of weapon. They came from Germany but their most famous applications were used by the Spaniards who hired them. It was also not as much 'a sword' as it was a combination between a polearm (without pole) and a sword. 

Arguably I wouldn't say it's a very efficient weapon in smaller scale combat - the weapon seemed most efficient in combination with pike (and shot). But there is application of it in duels, although admittedly I am unsure of how well they fared. I'm not sure on this, but I think the zweihander is considered the appropriate weapon for a lot of late-medieval sword arts books. 

Now here's the kicker - the weapon was only efficient in [i]trained[/i] hands. As in, you can give a peasant a sword and a shield and he'd be able to cut down a few men, but if you gave him a zweihander he'd be dead before even swinging it. It's a professional's weapon. That's where it power was but also it's drawback in my eyes. Due to the small amount of zweihander experts available, you'd never be able to amass an army consisting of them. They were more supplementary expert troops than 'main battle troops' so to speak. 

So, while they were effective in battle, there were also very few of them. If I had to choose between arming 10.000 peasants with swords or hiring 2.000 landsknechts for a larger sum of money than the peasants cost me, I'd probably arm my peasants, mostly because that's almost free (granted that I have the swords) where as landsknechts and mercenaries were historically unreliable and prone to looting and pillaging cities if they weren't paid [i]precisely[/i] on time.

The weapon itself is dope.