Watch it twice
All I can say for this film (and most of Tarkovsky's) is to watch it twice before passing much judgment on it. Whenever I first watch a Tarkovksy film, I often don't find it as alive as I hope for...there’s always a lot of silence and obscure talk whose point is hard to fully grasp. After watching it a second time, though, I find that these slower moments or dense dialogue is where the true power of the film lies. The second time through I always find myself much more able to inhabit the characters, and really start to understand what they are thinking and talking about (and without this, half the film is missing). The first time through sort of seems like a test run, in order to pick up on the main themes and meanings, so that you can then apply these in more detail the second time through.
After the first viewing, I was hard pressed to see it as one of Tarkovsky’s better films, but now I find it hard to argue that any of his earlier firms are better.