Influential
This movie seems pretty influential to me; Indiana Jones, Mission Impossible 2 and Casino Royale/Quantum of Solace and the Bourne movies all seem to have taken something from this film.
Clint doing all his own stunts in real environments I think was pretty rare in the 70s as well. Look at the James Bond films from this time, entertaining as they are, to prove my point.
Highly underrated.