There is a lot to grasp the first time watching it as the Dune universe is deep and there are multiple factions that have their own objectives while being manipulated behind the scenes by the emperor of the universe to keep his political power in check.
The extended version of the film helps to explain who the players are in the movie and their goals which in the theatrical version required multiple viewings to get a grip on if unfamiliar with the book the film is based on.
If you plan on watching I would recommend the extended cut as it will help greatly to understand this masterpiece the first time out as it is a cerebral type of film with concepts of scuence that were unfamiliar to audiences of the day. In comparison Star Trek was taken off the air for being too cerebral and it ended up making history for 50 years now. Dune will only grow in popularity as time goes on and the critics who trashed the film will hopefully never be taken seriously again when looking for a film to see.
I simply can't recommend this movie highly enough. The cast and scenery are, if nothing else, a tour de force unseen in modern cinema.
Unfortunately, even the dream cast could not help this film. Kyle MacLachlan's screen debut was quite promising, Sean Young and Sting also did pretty well. The casting was pretty good in general, but the movie just failed to deliver. Even as a director's cut.