Other reviewers have covered important points, but the essential quality of this film has only been danced around by a few.
'Stalker' is not to be watched, it is to be experienced. Those who find it too long or boring are standing outside the experience, looking on. Stalker is like an ancient spiritual or healing ritual. It offers you the opportunity to enter into the process and by joining in at each step, allowing the carefully crafted pacing to work on you, come to a point of transformation.
Each person has a different understanding of this film because what's in the Room is different for each of us. Every one of us has a deepest wish, usually unconscious, around which our entire life revolves. It drives all our decisions and relationships with people, things, society. And as the film makes clear, our goading wish is quite often not what we think it is - and not what we want it to be.
Entered into in the right way, this film can bring you to a new understanding of yourself. You may learn something you wish you hadn't. That's what happened for me... but I'd rather know. Others may choose differently. Others may choose not to even enter the journey into their personal Zone, in which case this film will seem long and boring, like someone else's dream.
Experiencing this film was a pivotal point in my life. It profoundly changed me. Like ancient rituals, this is what art - real art - is supposed to do, and Tarkovsky is the greatest master.
If anyone would like to discuss this film with me, please email.