Analysis of the Video:
While it's a good way to purify a large amount of water at once, it does pose some problems in that it might be difficult getting the water into drinkable containers (that need to be clean as well) after the sun has done it's work.
It's also not very portable when the bag is full, so it would need to be done at a location that one is going to be at for some time.
A few good ideas do come to mind from this demonstration however. One is that a smaller version a food saver bag would be a great way to do this. Not only would it be easily portable when rolled up and not in use, but also when full of water. A rolled up plastic bag doesn't take up much space in a BOB for instance, and it would be able to be used for other things as well if necessary. Having more than one function for equipment is always a great bonus.
Another idea that comes to mind is that this would be a great idea to not only purify a large amount of water at once, but also heat it up as well. I'm thinking taking a bath here. You wouldn't have to worry about getting any water borne disease if any of the water gets in your mouth, nose, or eyes since it will be mostly clear of harmful germs.
Laying the bag on top of something reflective, or black will help heat up the water faster and to a higher temperature which also kills more pathogens.
Also remember that for the SODIS process to work properly the water needs to be less than four inches deep, which this bag method seems to accommodate.
What are your thoughts?