I don't know if my interview was slightly different or anything so take my words with a grain of salt. My unstructured and structured were pretty much all the same. I found the past few years' interview questions online somewhere (I think it was on Facebook through a SDN page) and both unstructured and structured questions pretty much came off of those, or were very close. I didn't get any ridiculous questions, some curve balls but nothing terrible. None of the answers I gave were ever followed up with other "unstructured" questions. Essentially it went like this:
We have 20 minutes, we'll ask you some "structured questions" and some "unstructured" questions after. We cannot answer question about the questions or elaborate on them, we can just repeat them.
-- about 12 "structured" questions asked/answered
Okay, we will now move on to the unstructured questions
-- probably about 4-5 questions, none of which I was asked to elaborate on my answers or asked questions relating to the last question I answered.
Again, maybe my answers were satisfactory or sucked so bad they didn't want me to elaborate, so take my example with caution. Overall I was fairly happy with my interview, it takes some stress away knowing that it's not actually the people sitting there making the admissions decisions, but at the same time you are at a disadvantage since you can't "read" the person making the admissions decisions as you could with a one-on-one interview.
Also, I would say don't over prepare. Due to circumstances I had less than a week to prepare for my interview, and I actually think this helped me a lot. I knew what I wanted to stress about myself going in, and had some general (not detailed) answers prepared for questions I thought I might get. Figure out what is important for you to stress that may not have come out strong/clearly in other parts of your app and make sure you hit those points by either answering direct questions, or working the answer into another question that is somewhat related. Remember, you have no idea what questions you will be asked, so you may have to answer in a way that gets your point across even if you have to twist the interpretation of the question a little. But, being that they cannot clarify questions this works to your advantage as you can twist what is "meant" by the question.
I hope this helps, I got on a little bit of a typing spree there
Good luck!