they act just like a port they can be tuned by changing the weight added to the cone.
they save airspace since they arent very deep and allow you more design flexibility.
general rule of thumb is if you have a 10" sub u need a 12" radiator but this is not always necessary as long as the radiator can displace the same amount of air or more than the sub which is why boston's radiators are the same size as the speakers they are matched with.
personal experience they work good for smaller systems up to about the 145db range after that i cant say ive ever seen a passive radiator system get louder than that.
biggest downside to the radiators is generally their price