I think use of technology (like YouTube videos) is good way of starting a lesson. it get student motivated even though they probably don’t what they learning but visual can some times effect students cognitive think and get them relaxed before the actual lesson. but in doing we have to be careful about the length of the visual we are presenting because after a while students will start losing their focus. And, also we have to be careful about the content of the visual that, is it presenting what the lesson is about or its just for the entertainment.
The article on eduTopia is worth reading in order to harness the power of YouTube in the classroom.