Yes, the effects were impressive when it was made but that's about all. The story is very weak and the script is-especially the dialogue is beyond horrible and elementary.
Not only is the overall dialogue terrible, but the only female, Rachele Welsh's dialogue seems to have been written by a man whose never met a female in his entire life.
I'm far from being a feminist, but this film is a disgrace and totally disrespectful to women. Welsh's character is nothing but a pretty nurse capable of only asking questions and uttering monosyllabic responses-if not single words.
And, like everyone else, her lines are insulting to viewers (even in 1966). Pay attention to how often there's silence-as the characters just look around-as they're embarking on an AMAZING-never-done-before human feat. It's truly a Fantastic voyage yet none asked a single question: about the shrinking procedure, or what to expect...
They have no dialogue not because they're amazed at their surroundings, but because the (bad) writers simply had no idea what to write-for them.
The guy hired for "security" initially says he doesn't want to go, but willingly does after a five minute meeting in which there was no discussion about what effects the shrinking has on humans or anything else any human would have asked under similar circumstance. He simply smiles and goes to the "sterilization" chamber.
Putting the SFXs aside, I give it (at best) two stars.