Congratulations notorious725!
As for advice, my best advice is to get there a day or two in advance and get the lay of the land.
I sprang for a hotel at the Four Points Caguas (the only hotel in the city), which was SUPER convenient since it's just down the highway from the school. I also got the chance to talk to one of the students (who happened to be from my undergraduate alma mater) and whom I was somewhat acquainted.
Honestly, I learned more from her than I did from the staff (she was nice enough to give us an unofficial tour), and gave us a very frank and honest assessment of the school from a student's perspective--all DESPITE the fact that she wasn't a formal tour guide or anything--she was just a nice girl who felt like showing us around.
My best advice for the INTERVIEW, though? Read up on San Juan Bautista and its accreditation. One of the questions the asked in my interview, for example, was "What are SJB's accreditating bodies?"
(Answer: the Liason Committee for Medical Education, the Middle States Association, and the Puerto Rican health administration, whose formal name escapes me at the moment). I missed the last one, but they were really impressed that I could rattle the first two off (esp. since the girl I was with didn't know. Hopefully, though, that didn't hurt her).
They also asked some fairly tough questions: like, "What is your opinion of gay marriage?" They don't actually care about your politics, mind; rather, they want to know how you arrived at that conclusion. For example, I said "Well, I support it for reasons X, Y, and Z; the only possible exceptions might be A, B, or C, but I don't find those particularly compelling for reasons D, E, and F."
They also ask you to write an essay in your non-native language (e.g. if you speak English as a native, they ask you to write in Spanish, or vice versa), and the conversations of the interview started in Spanish, but flowed freely between the two.
Like most things in Puerto Rico.
The essay itself was similar to the interview; your ANSWER didn't matter as much as your thought process behind it (and, more to the point, your ability to write in your non-native language). For example, my prompt went something like this:
"A 13-year-old girl comes to you, and is very clearly pregnant. She's come to you asking for an abortion. What do you do? Do you tell the parents?"
Oh, but very important! If you rent a car to Caguas, be VERY careful driving. People in PR play kind of fast and loose with traffic laws, and on the way in from the airport I saw three accidents (fender-benders, no injuries, but still). Oh, and be sure to carry plenty of quarters in your car, and have them readily available. There are a bunch of toll roads between Caguas and San Juan City, so that'll save you the embarrassment of running up and down three lanes of traffic trying to get change for a dollar.
Oh, and whatever you do, DO NOT use a GPS in Puerto Rico. Mine told me to "turn left," off of a bridge, so you'd be better off using a map of Middle Earth, for all the good it'll do you.
Either that, or I ROYALLY pissed someone at Garmin off...
But yes, aside from that, just have some fun with it! The staff there are SUPER nice, and Sra. Sanchez (the admissions officer) is almost impossibly nice, sincere, and helpful. If you have any questions, she'll be more than happy to answer them.
Hope that helps!
Oh, and this is a lovely website with more information about Caguas. It's all in Spanish though (but that shouldn't be a problem, yes?):
http://visitacaguas.com/informacion_comollegar.htm