Despite being one of the key dozen or so science fiction writers of the 20th Century, relatively little of Robert Heinlein's work has been filmed and, with the exception of Starship Troopers (turned on its idealogical head), nothing from the 60s onwards, when his work started to carry more serious subtext.
The Puppet Masters, a book from the 50s, is essentially fairly lightweight albeit the invasion motif is similar to that of much 1950s science fiction in echoing a fear about the spread of communism - indeed, the parasitic slugs in Puppet Masters form a single entity - a commune if you will - and are therefore a fairly obvious analogue for the red menace. The novel was also fairly saucy for its time, with large chunks of the population having to peel down to their skivvies and beyond to prove that their clothing was not hiding the alien slugs.
Well, as usual, Hollywood doesn't like to present the naked body to the viewing public, so the protagonists in this film version of Heinlein's story encounter more difficulty than they need to, simply by virtue of everybody keeping their clothes on. Apart from that, it follows the main sequence of events fairly faithfully, albeit updated to be more contemporary.
Donald Sutherland is a serviceable "Old Man", albeit quite different to how I imagined the character from the book (which hardly matters). There are several well-known albeit minor names in the secondary case, but the main leads are taken by Eric Thal and Julie Warner. Neither has had major success before or since (Warner had a lengthy run in Family Law) and, to be perfectly frank, this movie gives a good idea why - they have little charisma, and no chemistry despite the fact that they supposedly fall in love during the film The story demands lead players who don't come across as insipid, but it doesn't get them.
There is relatively little in terms of special effects, but what there is does the job. The major effect is the slugs themselves, and they are pleasingly unpleasant..
There is some offensive - and entirely unnecessary - use of the F-bomb. I don't mind it when it's there for a purpose: here it service no purpose whatsoever. It doesn't advance the plot or illuminate character, it simply makes an unpleasant comment even more unpleasant, and it could have been omitted entirely without any detrimental effect on the movie.
With some promotion, this movie could have been rather more successful than it was, albeit it came on the heels of what might have appeared to be similar epidemic-type stories. Perhaps Heinlein's time in movies is still to come.