I actually just got back from my UTMB interview today, and I'm 99% sure they look at your entire application (GPA, MCAT, letters of rec, personal statement, interview feedback, etc). I know interviews carry a lot of weight, but I don't think any school bases its decision on that alone.
As for my experience, the day was really enjoyable and informative. Both of my interviewers were very nice and easy to talk with. Nothing high stress at all. They had my entire application to review, but I got the feeling they didn't have my GPA or MCAT scores because one of them asked if I had done pretty well in my classes. They must keep that info separate so it doesn't influence their impression of you. I think it's a pretty good idea. Well, good luck with your interview.