I've heard that the auto inflate models will occasionally inflate if used during a down pour. That's why I went with the manual, pull the lever type. Just have to be careful that I don't pass out before I fall in.....
That was sometimes the case with MIT models. They have a tablet that dissolves in water which sets it off. Sometimes in heavy rain or wet bottom of a boat, they can go off prematurely.
The newer HIT models have to be submersed in 4" of water to inflate. A little more money but no need to worry about it going off in wet conditions, and nice to know if you can't get to the chord it will go off if you go overboard with the autoinflate.
Also incase anyone wasn't aware, if you have an inflateable PFD on your boat but are not wearing it, it does not count as having a life jacket on board if you get checked. It has to be worn to count.