A nice first effort by what would soon become a legendary director Reading the other reviews, I tend to agree to what other IMDb users say.
The film is indeed timeless. Average directors usually try to stick to the trendy cinematography tricks of the moment. Great directors try to invent the next trendy tricks! Apart from a few very 70's- cliché zoom-ins, photography, transitions, and actor direction could easily stand with today's standards.
One reviewer compared this movie to other UK's legendary director Stanley Kubrick timeless masterpiece: Barry Lindon. They both share the same main theme (honor during Napeleonian wars). They do have a lot in common, Barry Lindon was produced 2 years earlier than this film, so you can easily believe that Scott "borrowed" a few "ideas" from his mentor.
So, why just a 6 out of 10 then? Well, the script is poor and a bit boring. Blade Runner was inspired by a short novel, so was this movie, but unlike Blade Runner and Barry Lyndon, Duellists did not succeed in transcending its original medium.
In a nutshell, an enjoyable film to watch, strong recommendation to all Ridley Scott's fans, but a very optional view for regular moviegoers.