What kinds of information and insights did it give you about the usability of the prototype?
This practice gives you first hand feed back from your observation immediately. Sometimes, the verbal and non-verbal behavior would work together to give out information. But sometimes, the later one might comes out earlier as the user is looking for a conclusion before he/her can say something. And to some extent, we designer should make the process more enjoyable: make user reach their target easily.
This practice also reminded me of conversations when I talked with my urban studio tutor about my assessment. I want to get as much information as possible from my tutor, so what I did was not only hearing what he said, but also his non-verbal behaviors, like facial expression, which shows the information behind and your tutor may not choose to tell you at that moment.
What aspects of the technique worked well or were frustrating?
As an observer, you can get some feedback from the user when he/her interact with a piece of technique. But the outcome varies not only because of your observation ability, but also the quality of user. In addition, users might just keep silent and make unconscious behaviors instead of what “think-aloud” requires.