It seems to me that I see Tarkovsky' movies differently from many other people. For me this film is not "too long" or boring. For me this is one of the best movies ever made.
Western culture has a very long tradition of film-making. Usually typical western movie is focused on "story". (Of course - not always) The sharpness and tension of the movie are achieved by the big number of cuts or by the fast varying of shots or by the sudden varying of plans or by some surprising angle of camera etc. Tarkovsky don't like cuts. The number of cuts is minimal. His camera is moving like in dream (Bergman envied Tarkovsky for that), it has no angles at all. Colours are pale, "dirty", very tender, soft, almost black-and-white.
In a typical western movie dialog is followed by the camera. Picture is illustrating text and is subordinated to it. In Stalker text and visual image are coexisting, cooperating with each-other. Both are moving on their own ways but at the same time, somehow - harmonically. Text and picture are not subordinated, they are both independent.
Why is Tarkovsky using such a weird language? Surely not only because he wants to opposite the dogmas of western cinema. He has a positive message too. Audience of his films has to understand his films not only at the level of thinking or emotions, but at the level of much deeper consciousness. Therefore watching his movies means rather meditation than watching-TV-and-eating-popcorn. The purpose of Tarkovsky's films is to loose the mind of audiences, to wake it up to much deeper attention. So that audiences can simply watch and see.
Stalker is not an entertainment and is not supposed to be. It means there is no sense at all to watch Stalker, when you need some amusing entertainment. Stalker is a serious movie. It is very narrow-minded to evaluate movies on the assumption of entertainment only. Of course, we live in the world of movie-consumers, produced by powerful film-companies, demanding more and more and more exciting entertainment. Consumer doesn't understand this movie. For him it is big bore.