"There's a fire, sir." is one of my favourite lines in cinema, and indeed, books. Every time I watch this gem of a movie, that line alone makes the hairs on the back of my neck stand up.*The moments preceding this are also masterly in context.
This film is genuinely a text book exercise in how to make a film of this type work.
The claustrophobic element has never been done better. While the cast contains few if any A-listers, the acting is universally up to the task. Yes, in true Crighton style, the characters lack a little dimensionality, but that only helps to allow the movie to concentrate a little better on the situation. The pool of characters we get in the bunker is nicely differentiated, and each one brings a different element to the solution of the puzzle. There are few if any obvious scientific howlers, and while it's not quite perfect in that respect, it doesn't have one screaming at the mind-numbing inaccuracies they can get away with today after decades of dumbing down our education system (writing in England).
The access to the bunker complex is a "Man From Uncle" style delight; the story is not completely linear-there are changes of course in the plot (how rare is that today?); (SPOILER !)they persuade the authorities to A bomb the town, then find out they should not.
End of spoiler
The final sequences are very, very tense, and if the final ending of the organism is a rather anticlimactic, it's probably the more accurate for that.
This is one of Science Fiction's very best movies.
*There is one better line in cinema- "Night of the Lepus"'s "Run for your lives! There's a horde of killer rabbits coming this way !"