I bought the West Marine offshore auto/manual inflateables. These come equipped with an auto inflation system, a device to convert it to fully manual and a spare cartridge. When fully inflated, they provide 33.7 lbs of buoyancy - which is about double what a typical foam vest provides. They are considered typeV with type II performance when armed for automatic inflation, type III when armed for manual.
When properly maintained, the reliability and performance are excellent. I think the concerns of flapbreaker regarding reliability are unfounded and many military men and those in law enforcement entrust their lives to these (and similar) devices. While the coast guard aux member might not wear one, I beleive that many in the coast guard, navy, airforce and local law enforment do and have for years. The general consensus amongst users is that the increased comfort SIGNIFICANTLY increases actual use in the field and that the net benefit is positive.
Also it is important to note that the type V's DON"T count in your inventory UNLESS THEY ARE WORN. Hence, if you only have the Type V's on board, you MUST wear them to be legal. Since we got ours, we always have them on and the legal requirement is extra incentive. I bought the dual manual/auto ones since they contain a space cartridge and can be re-armed easily in the field if they unexpectedly inflate. The canister itself is of less concern than the dissolvable tablet that arms the firing mechanism. However both have recommended replacement schedules of every 2 years. The cartridges are CO2 and are very similar to those used in pellet guns. I suspect that the reliable lifetime of the cartridges (barring unexpected inflation) is actually much longer than 2 years.
The offshore (Type II rated) autoinflating life jackets support you in a very similar manner to other Type II non-inflatable devices. With 33 lbs of buoyancy, you will farther out of the water than with the vast majority of vests that people carry. Type II will also float you face up if unconcious.
As an aside, I also carry a Mustang bomber jacket that's a typeIII on board to wear on cold days/mornings.