The closed circuit rebreather has been called the most significant development in diving since Cousteau invented the aqualung, and with good cause. If you are a serious diver, the advantages of this system and the freedom it allows are worth the time and added expense of purchasing the gear and getting certified. Just be aware that this is a vastly different kind of diving from the open circuit diving you’re familiar with and you will be starting out as a newbie with a lot to learn.
In the most basic terms, a closed circuit rebreather takes all of your exhaled gas and retains it within the system in a closed loop. That exhaled gas is then filtered, refreshed, and recycled back for you to breathe again. Bubbles will escape from this system only during ascent and mask clearing. Closed circuit rebreathers maintain a constant partial pressure of oxygen (PO2) at every dive stage, so you’ll always have the best mix for every depth.
Adapting to this new way of diving may take a while since even the sensation of breathing is different; instead of the regulator noises and the feeling of high-pressure air being shoved down your throat, there is silence, and breathing is as natural as it is on the surface. Buoyancy control is also different; when you exhale, that air is not released, it is shunted to the counter lung, so there is no change in buoyancy. This is a bigger issue during ascent where the expanding air can cause a quicker rise than you’re used to, but most systems have a buoyancy compensator that functions like a traditional BC.
The majority of divers who want to transition to a closed circuit rebreather do it in order to have a closer interaction with marine life; sometimes for purposes of recording amazing videos, sometimes to better study specific creatures, but other times, just for the experience. Unfortunately, the acoustic waves produced by exhaled air can trigger skittishness even in dolphins and whale sharks. Removing the bubbles also seems to remove a major barrier between the species. Despite these small debatable disadvantages, there is little doubt that closed circuit rebreathers give divers a new freedom underwater.