I've used them and still do in places where PFD's are required and at times when I'm sailing alone in
windy choppy conditions.
I have a PFD from Perception Kayaks that works quite well.
It will fit over a seat harness, but it's not as easy to hook in and unhook as without the vest.
Look for one that has more flotation in the front and high up so it will do the same thing as a
full USCG approved Type I PFD and turn you face up out of the water if you are unconcious.
Unfotrunately there are head trauma and life threatening things like heart attacks that you
can survive, if you float with your face out of the water and can breathe.