garfish is right pbo did a very informative report on radar reflectors, ive studied microwaves and radio frequenices and it was not bollox.... infact escluding active units, its trial and error....
all other radar reflectors return a signal along with your boats own signal, for want of a better way to describe it these wavey signals can sorta crash into each other on the return path, this can have the effect of putting the signals on top of each other and increasing your visibilty, however as the frequency has both positive and negative bits when they crash into each other a negative bit could actually attenuate your original signal and hence make you less visible or make it peak greater but for such a short time ships wont see it..
they are not all crap, it tends to be individual to each boat, any reflector could be great at improving your signal or dog poo on any boat, so its real trial and error, maybe find a mate with a radar and do a test w or w/o
There was a guy that lined his whole yacht with tin foil to increase his visibility and almost made himself invisible due to the angle of the reflected waves etc, they compared him with a non lined and very visible identical boat!! monkey!!