There are two types of auto-inflatable PFDs, the ones with the dissolving bobbin actuator, and the "hydrostatic" type which use some sort of pressure sensor to actuate. Both types use a CO2 cylinder (plus a backup "jerk to inflate" cord and a manual inflation tube).
The bobbin-type PFDs are somewhat prone to accidental inflation, caused by a good dousing from an errant wave, or as the bobbin gradually softens due to humidity and age. The hydrostatic ones require a few inches of submersion, and are triggered by the water
pressure. These are supposed to be just as reliable, but not as prone to accidental activation.
I currently have the Mustang hydrostatic type with built-in harness, and am very happy with them. I previously used the bobbin-type (with harness) sold by West Marine
and had no problems with accidental activation, but we have a pretty dry boat and I replaced the bobbins annually. The Mustang design is more comfortable than the older West Marine unit, especially around the neck, but I believe that the WM design has been improved in this regard.
Either of these do give you interesting tan-lines.
Spinlock has what I am told is a very nice auto-inflate PFD. It may not have U.S. Coast Guard certification
, but when I last checked it had the appropriate European certs.
Having the built-in harness is a great feature, and other than a slight extra cost I can think of no downside.