Tron had taken claim to the first all computer generated sequences. But since the objects in Tron existed in a symbolic world, they were not intended to look "real", so the techniques used were better referred to as computer animation. What is meant by modern CGI is the full rendering of objects by a computer into a scene that is meant to mimic real objects, which is what we are so used to seeing today. At the time, it was a very different kind of look, enough to leave an impression.
But it must be emphasized that the effects are just a sidebar. The real charm of the film is the way the story is told and the characters that are met along the way. Robert Preston does a wonderful job as Centauri, the cosmic salesman, in his last performance before he passed away. Is it science fiction? Well, it uses all the traditional elements of sci-fi, but never really takes them completely seriously. It's one of the few films in this genre that I can recommend to both fans and non fans and know they will probably have some fun with it.