I've rented from MIA a couple of times, never had a issue. Twice my car wasn't ready and they me if I wanted to wait or switch to another class, being that I always reserve prestige models I opted out to wait. Which isn't a big deal to me, unline other people who are always in a hurry. But as far as upgrading apperantly Hertz has a new system where you will get a Email letting you know what options of cars they have for you to upgrade to. Dont know if thats true or not being that I haven't traveled since late December, but I'll find out this weekend while I'm in Chicago. MIA has a gold upgrade section right next to the gold canopy and you could pick a car from there.