BCs

If you are using a BC that is more than five years old, you may be amazed by the improvements in comfort and function offered by some of the current models.
Fit is fundamental. Ideally there should be no slack or air space between you and the BC. This will only translate into a cylinder that flops about on your back once you are in the water.
On the other hand, a too-tight BC will restrict your breathing and exert pressure across your chest and diaphragm. A correctly sized BC will not need to have its adjusting straps pulled right out to the stops to achieve a good fit. There should always be some room left for the adjustments you will want to make when using it in different conditions and with different suits.
The specification given for each model often includes the denier rating for the nylon used in the construction of the bag and other parts of the BC. It is a good idea for the outer and exposed parts to be damage-resistant and a high denier count is useful here.
Some manufacturers use the same quality of material as is specified in bullet-proof vests carrying the classification "ballistic", but in that application it is used in several layers and usually supports the really bullet-resistant materials, such as steel plate.
The flexible parts of the BC such as the bag or air-cell need to be made from a more supple material with a lower denier rating so that they can inflate fully to give maximum lift without needing to overcome the stiffness of the material. Oceanic uses stretchy Bioflex kevlar material that allows the air cell to expand as it approaches full buoyancy.
A major contribution to increased comfort in recent years has been the availability of integral weight systems. Removing the concentration of weight from around the waist and re-distributing it higher, both on the front and back of the BC, generally improves the diver's balance.
Typically, about a third of the weight is carried in two pockets on the back of the BC and the rest distributed between two releasable pockets at the front. This system, pioneered by Zeagle and SeaQuest, has now been adopted by many other manufacturers.
Make sure the release systems work well and will stand up to repeated use. This is not because you are likely to have lots of emergencies but because it is often a good idea to slip your weights out of the jacket before climbing back into a small boat. A heavy set with a generous amount of weight incorporated makes an unwieldy package to pass up to a buddy.
But once you have tried diving without a weightbelt, you will resist any temptation to go back.
We can thank the technical divers for acceptance of systems that put the major buoyancy behind the diver, using a wing-shaped bladder instead of the more conventional waistcoat shape. This significantly improves the diver's swimming position while submerged, but is not especially suited to maintaining a face-up position at the surface.
Wings are well-suited for use with larger and heavier cylinder sets, as they position the buoyancy close to the main centre of dead weight. They typically give a cleaner front with good access to drysuit valves but less pocket capacity.
This is only partially compensated for by the provision of numerous D-rings for the attachment of other equipment. If you are not regularly involved in mixed gas diving, resist claims about all the D-rings fitted.
If you travel frequently you might appreciate a jacket without the conventional rigid backpack, allowing it to be rolled up. Travellers' BCs tend to presume warmwater destinations where there will be little need to compensate for major wetsuit buoyancy loss, so they will typically have lower lift, below 15kg.
Women are increasingly well catered for these days, with special designs to give a comfortable fit without the usual tension points across the chest. The SeaQuest Diva has been joined by the Aqualung Spiro Eva, Scubapro Venus, Sherwood Luna and Forte Sirene.
Some BCs achieve their maximum buoyancy configuration by combining two bladders, or by adding a set of wings to a conventional BC. While solving the lift problem this introduces the complexity of a second inflator and the need to control two separate buoyancy chambers during the dive. Appropriate training is advised.
Because the BC is so central to the diver's equipment system it usually has to provide for carrying all the other accessories. The trend away from voluminous pockets has been partly arrested by the introduction of roll-up pockets that give useful capacity when you need it, but roll out of the way when not in use.