The Meat
All you really need to know about They Live is this:
There is a moment when Rowdy Roddy Piper, the WWF wrestler, walks into a bank with a gun and says "I have come here to chew bubblegum and to kick ass, and I'm all out of bubblegum," at which point he begins shooting the hell out of the place because his magic sunglasses allow him to see most of the customers in the bank are actually skull-faced aliens masquerading as human beings and brainwashing us through advertising.
Cult immortality: achieved.
The Math
Objective Quality: 6/10
Bonuses: +1 for (see above paragraph); +1 for finding a way to mash up The Grapes of Wrath and Noam Chomsky in an alien sci-fi picture; +1 for offering legitimate social commentary that rings true 25 years later; +1 for its place in the canon of vintage John Carpenter movies; +1 for the last shot in the movie, which I won't spoil here.
Penalties: -1 for the seven-minute back-alley fight scene between Roddy Piper and Keith David (I know a lot of people like this, but it is literally seven minutes long); -1 for Roddy Piper's immediate, complete, and never-questioned transformation from mild-mannered day laborer into unstoppable killing machine.
Cult Value Coefficient: 9/10
[See explanation of our non-inflated scores here.]