A balanced regulator is a regulator that has been designed to maintain its ease-of-breathing regardless of depth or tank pressure. In contrast, a diver breathing from an unbalanced regulator will notice that breathing resistance (the amount of effort it takes to draw a breath from the regulator second stage) increases with depth or as the scuba tank pressure becomes lower.
A balanced regulator has several advantages. Divers who intend to do deep recreational or technical dives will benefit from having an easy-breathing regulator at depth. However, unbalanced regulator designs have improved so much in recent years that the average diver may not notice a difference between a balanced and an unbalanced regulator on dives shallower than 100 feet. Breathing resistance is also related to exertion, and the more heavily a diver breathes, the more difficult it becomes to draw a breath. Many divers report that balanced regulators deliver air more easily than unbalanced regulators on dives requiring exertion.
The main disadvantage of a balanced regulator is price. Balanced regulators are generally more expensive than unbalanced regulators because the design is more complicated. This may also increase the price of servicing a balanced regulator. Because the design is more complicated, there are more parts to service (and more parts to break).
Some divers prefer unbalanced regulators for the very reason that others dislike them. As the tank pressure gets low, it becomes more difficult for a diver to draw air from an unbalanced regulator. The increased breathing resistance is a warning that the tank is getting low on air. If a diver routinely checks his submersible pressure gauge, however, such a warning should be unnecessary.
Each diver must decide for himself whether the ease of breathing very deep, or with very low tank pressure is worth the significant increase in price for a balanced regulator. The answer will vary with the type of diving and the individual diver.