I've rented at MIA a gazillion times by now and personally, I find it one of the best Hertz locations ever. I have never encountered any problems; I'm a Gold member and their Gold Canopy works immaculate. My name is always on the vultron board, the right car is always in the parking lot and I always get what I reserved, pending it's a Prestige, Fun, Green or Adrenaline Collection.
From I reach the rental center and goto the parking lot for Gold members, it takes 5-10 mins for me to pick up my car effortlessly and drive out.
I've only had one negative experience with a Mercedes-Benz E-Class being very dirty, 15 mins later I was driving off in a new E-Class.
The lines at the regular counters are sometimes endlessly long, even at the Gold counter if you choose to go there for some reason. But it's like that with any of the companies as it's a big rental centre with over 6500 cars.