I cannot speak to the effectiveness of the stun gun in the Youtube. It allegedly puts out 300,000 volts. Notice that the focus of the photographer is to capture fun reactions - and his extended video is almost entirely about the photographic process, camera, equipment, lighting, etc.
I can speak to the 100% effectiveness of the Taser (R).
http://www.taser.com/products/law-enforcement/taser-x26p
In 2010 or so I was in a military course and we were certified on the Taser x26. In a group of about 50 big, strong, brave Special Forces and support staff Soldiers, we had to shoot each other. All 50 (including myself) dropped like sacks of rocks for the 5 seconds of the pulses. There was no - meaning zero - ability to laugh, resist, fight back, or do anything other than lock up and fall down during the duration of the charges (5 seconds, 19 pulses per second, total of 95 pulses in 5 seconds). Zap, zap, zap 95 times in 5 seconds. It was the most horribly painful experience - it felt like what you imagine being electrocuted would feel like. The effects were largely worn off immediately afterward. The shockee felt 'weird' but could otherwise function within moments after the shock with no issues.
Watching others ahead of you only added to the horrific anticipation. The x26 - for APPREHENDING an individual or gaining compliance while making an ARREST is an excellent short range tool.
It is not well suited for civilians - we don't have backup, it is cumbersome and short range, no fast reload ability if you miss (and they are not very accurate - even at 10 feet on a still target, there were some people who managed to miss), and we aren't making arrests or apprehending others.
Hence, I would not carry or incorporate one for myself as a civilian. If there is a threat (perhaps a non-lethal but still serious threat, or an animal that I don't want to shoot like a bear or neighbors dog), I may use non-lethal pepper spray if I have it, allowing me to escape or avoid further escalation. Pepper spray - in my experience - is quite effective at debilitating nearly every person. Not to derail this, PS does have limitation on distance, wind, indoor/outdoor considerations, etc.
I assume this is directed largely at the female or anti-gun population who think a stun gun is a good alternative. We see these in gun shows regularly. I agree they are a poor alternative. If there is a lethal threat then the appropriate response is lethal IMO, and that means a firearm in most cases due to their proven effectiveness.