I went to the one in Phoenix last year and I thought it was great. They had us do 2 interviews, and after each one they gave us feedback and constructive criticism. After that they fed us and talked to us about the admissions process, if I recall correctly. The feedback is great, because they are completely honest with you. You won't get that on the actual interview trail. In fact, the feedback you get at actual interviews is at best useless and at worse serves as a false hope. Case and point, I was told unsolicited that I did "fabulous" at one interview and "fantastic" in another, and I did not get accepted at either of those schools.
As for preparation, I'd say try to prepare for it like you would a real interview. Take a look at your application, and see what aspects of it you'd like to bring up to them. Keep in mind that since this is a closed file interview (like the actual UA interview) you'll want to provide all the details that they wouldn't otherwise have. Also, I'd suggest looking at some information about the school, so you can have a specific answer to why you want to go to UA. Other than that, I don't think there is a whole lot you need to do to prepare. Just relax and treat it like a real interview, so you know that the feedback you receive is representative of what an actual interviewer would think.
