There are a few interesting points from this video that I wanted to highlight. First, this effect really demonstrates the role our brains play in constructing our experience of the external world. The brain takes in stimuli from the various senses, and then seeks to integrate those stimuli into a cohesive whole which is what we then experience. We do not directly access the world through our senses, rather our brains construct the reality we experience. A second interesting feature of the McGurk effect is that it demonstrates the primacy of visual stimuli over auditory stimuli. When there is a conflict between the two (as when we hear the word "ba" but see the speaker moving his lips as if he were saying "fa") our brain privileges the visual and corrects what we hear to match what we are seeing. This is closely related to the audio pareidolia I discussed in my previous post where the captioning really assists one in hearing what is being sung. A third interesting feature of the McGurk effect is that this is a perceptual illusion that we can't avoid hearing. Even if we know that the word being said is "ba," when our eyes see the lip movement for "fa" we can't help by hearing "fa." This demonstrates that even when our perceptual faculties are functioning perfectly, we can still succumb to systematic unavoidable errors.
The take away from this (as with all perceptual illusions) is that we should recognize the unreliability of our experience and the experience of others. This is why when someone relates some incredible, unbelievable experience that they had, we should be skeptical about the veracity of that experience. This is also why anecdotes and eyewitness accounts are so unreliable.