Like Stephen King, Robert Heinlein is an author whose work is frequently adapted into Hollywood screenplays. Also like Stpehen King, the results of adapting Heinlein's work into the format of film rarely live up to the potential set by the material. The major difference between the two authors is that an adaptation of Heinlein usually gets better the further it strays from the source, so long as it sticks to the spirit. Ironically, the Verhoeven/Neumeier adaptation of StarShip Troopers remains the best that Hollywood has come up with so far.
The problem with The Puppet Masters as a film is that after a well-constructed setup in which the aliens are made entirely plausible, even somehow real, it descends rapidly into yet another action/sci-fi hybrid in the vein of Predator or Aliens. This in itself wouldn't be so bad, except for two things. One, Heinlein's stories were always intended as the exact opposite kind of science-fiction. Two, the mixture of the two elements leaves the film unbalanced. The second half feels very disjoined from the first.
The scientific concepts shown in the film are not perfect, but they give it an atmosphere that many horror films of the era lack. However, the science of the film is not the only key to making a great story. Where it falls down is in the Transition. The Transition from one moment to the next leaves gaps of logic that disconnects the audience from the story. The questions of how on Earth one thing got here or there are never adequately answered. Telling us that something escaped never works as well as showing us, and unfortunately, this production of The Puppet Masters never does enough of the latter.
The acting doesn't help matters a lot. Donald Sutherland is very competent as usual, but the script gives him little to distinguish his character from the other several dozen that he's played in an almost identical manner. Julie Warner stands out as the character who grabs the viewer's attention, by fair means and foul. Eric Thal, on the other hand, is like Keanu Reeves. You could replace him with a wooden plank that has a mean face painted on it, and nobody would know the difference. Richard Belzer stands out in an almost-wordless cameo. Anyone else in the film never gets the chance to distinguish themselves.
The second half and inadequate characterisation of the film would have been easy to overcome if only one of them were present. The only problem is that third, fourth, even fifth problems are added. One major problem is the sheer volume of exposition that is delivered through dialogue. It is one thing to tell us how character X felt sensation Y at moment Z. To do this without flashbacks or recapping footage from new angles is a difficult ask at best, and with a cast of this calibre, it simply shouldn't be done.
Ironically, the concept of The Puppet Masters was done slightly better in a direct-to-video sequel to StarShip Troopers. The concept of an alien parasite using a host for a sinister purpose was also done better in The Thing. If you've already seen those films, then The Puppet Masters is worth checking out. If not, I'd recommend checking them out first. A 5 out of 10 from me.