If you want to dive with the McDonalds's of Key Largo/Upper Keys dive operations, you will pick Rainbow Reef. That is how the local dive community refers to this operation.
I was a former intern that got trapped into the advertised offer of working for free for six months with housing provided and Rainbow would sponsor to get me to my Dive Instructor. Six months of free labor traditionally turns out to be 9 months of 60 hour plus weeks of free labor, with no guarentee they will actually pay or get you through all the classes and dives required (100 minimum dives) to become an instructor.
The reason they offer "free" guides is at the expensive of these interns who willing signed up for free labor (until they realize they can actually get paid and work at regular dive shops in the community and get free or highly discounted classes and still get their instructor rating but actually get paid doing it). The interns are awesome people that got wrapped up in the wrong dive shop and now need to see it through.
Some love it at Rainbow Reef and continue to stay on as an employee but others realize the treatment and quality they are missing out on at other professionally run dive shops in the area, and the best dive instructors in the Keys go elsewhere for employment.
Rainbow Reef is a revolving door of staff and interns (some of which should not be caring for divers safety) and the owner is laughing all the way to the bank at the expense of the interns that make up a majority of his workforce.
The clients are also missing out on much better dive experiences and dive operations in the Keys because they get lured in by great reviews, free guides, and the like.
Always expect crowed to the limit boats, rushed dives, being told to arrive at the dive shop at 7:45A.M., with the staff inside looking at 20 people locked out of the shop (while it is raining outside) and telling you to wait until they are ready to open the door (don't tell clients to come check in at 7:45 A.M. if you aren't ready for them at 7:45 A.M. and then make your clients feel stupid for getting there on time).
If you want to experience real diving in the Keys please head to Horizon Divers or Quiescence in Key Largo (they go to all the same locations that Rainbow Reef does and a much better and relaxing dive experience), Florida Keys Dive Center in Tavernier and Keys Dives in Islamorada. All of the listed shops are well liked, treat their staff well, focus on safety, give back to their communtiy, and help with reef clean up efforts and other volunteer activities.