Some quotes from Roger Greenspun's Oct 8, 1971 review of French Connection:

"I don't mean that they are not exciting. The French Connection is a film of almost incredible suspense, and it includes, among a great many chilling delights, the most brilliantly executed chase sequence I have ever seen. But the conditions for the suspense (indeed, the conditions of the chase—to intercept a hijacked elevated train) carry with them the potential for failure not of this particular action, but of all action in the great doomed city that is the film's real subject. From the moment, very early on, when Hackman first pistol-whips a black pusher, you know that the world is cursed and that everybody playing out his allotted role is cursed along with it.
In a more pretentious and less perceptive film, destinies might have turned tragic. In The French Connection they become all but invisible. The whole movie has slightly the look of being background material, or maybe excellent pre-credit material, for another movie. It moves at magnificent speed, and exhausts itself in movement. The central characters repeatedly appear as if out of the city's mass and then disappear into it again—a superb conception for an action of difficult pursuit, but one that never allows the luxury of personal identification.

That is why only Gene Hackman surfaces as a character, although there are the fragments of many good performances—seen as if across the street, outside the window, or at the other end of the subway platform. There are also faults: a murder too many, some shaky motivation among the bad guys, a degree of coldness that perhaps even exceeds the requirements of the cold intelligence that controls the film.

But The French Connection is mostly a credit to everyone who helped shape it. This would include Ernest Tidyman, who wrote the screenplay and who also wrote Shaft; Owen Roizman, the cinematographer; and William Friedkin, a director whose previous work (The Birthday Party, The Night They Raided Minsky's...etc.) may not have prepared anyone for the excellence of this."