For the first few hours Doctor Zhivago is an absolute masterpiece. David Lean's depiction of one of the more tumultuous times in Russian history is breathtaking in its scope and touching in its story of a family trying to make it against large odds.
For me, the problems crop up when the movie becomes less of an epic scope and more of a personal romantic tale, detailing the relationship between the titular Zhivago and his mistress Lara. It seems as though Lean was so focused on creating his grand tale of the Russian revolution and World War One that he forgot to lay believable foundations for the Lara-Zhivago relationship.
Based on what you actually see in the film a one week fling seems more reasonable then what the film actually gives you. Still, outside of this shaky bit of story, Doctor Zhivago is a towering achievement. Omar Sharif is great, it has some very creating directing and it is one of the most beautiful films ever put out.