This is so much more than a 'monster movie' and more of a social commentary on the negative effects and dangers of nuclear war. Godzilla represents the nuclear bomb that destroyed Hiroshima and Nagasaki.
The original Japanese version is the one to watch whereas the American version is sanitized and avoids much of the underlaying social aspects the director is trying to depict in a country just begining to stabilize after WWII.
None the less, this is my favorite of all the kaiju genera as it is original, moving, and thoughtful in scope. There are many levels to this film and all will find something to enjoy.