What kinds of information and insights did it give you about the usability of the prototype?
Recording the users’ words and observing their facial expressions are like gathering the first-hand information about how the users feel toward the prototype. When they found the prototype was easy to use or they experienced some kinds of confusion or found something that was redundant, the observer would know immediately. Observing the users while they doing the task gets a real-time feedback.Not like only handing them a questionnaire after the task,since they might forget some points.
What aspects of the technique worked well or were frustrating?
Well,I think they all worked well.By combining the words and facial expressions of the users could give the observer general insights about their user experience. However,when noting down the words,I did miss some of the users non-verbal behaviours.