They aren't "cool" off-shore lifejackets, they're auto-inflating (usually) PFDs. (There are manual-only versions, as well--such as the one your URL points to.) The advantage is they aren't bulky and uncomfortable during normal wear.
Personally, I don't see much sense in the manual-only versions. If you get knocked off the boat, and you hit the water unconscious, well...
The auto-inflating ones have a lanyard for manual activation. Both the manual and auto-inflation ones have a mouth tube for really
manual inflation. (And to keep the pressure up if you find it has a small leak.)
Probably the top of the line in these, right now, is the Mustang with integral safety harness and "hydrostatic" auto-inflation release. But they're also very
expensive. The advantage to Mustang's "hydrostatic" auto-inflation release is that, allegedly, simply getting wet won't trigger it. The "trigger" has to be submerged in at least six inches of water, IIRC. The "normal" auto-inflation trigger has a "pill" that, when it gets wet, disolves and releases the pin. There have been the occasional report of these triggering when the PFD got extremely wet for an extended period. Others report not having had that problem.
My wife and I have the Mustangs with hydrostatic release, but w/o the integral harness. To be completely honest: The only reason we bought those was the store had them mis-marked, so we got them for the price of the non-hydrostatic Mustangs. We would've been hard-pressed to justify the real cost.
Either system is the same in that, yes, once it triggers, you have to replace the inflation mechanism. I understand that isn't cheap, either. Also, even if they're not activated, they have a "life span." You need to replace bits of them (just the trigger mech?) every few (?) years, I believe.
I'm not sure, but I'd bet the inflation mechs and gas cartridges are proprietary to each manufacturer's product lines.
Had we not lucked into the deal we did, I would have satisfied myself with either SOSpenders or Mustangs with the "normal" auto-inflation system. If I had my druthers, I'd go with the Mustang with hydrostatic release and integral safety harness