First, it seemed to me that the film doesn't capture the book as well as, for example, Solaris by the same Tarkovsky. I imagined the hero quite differently; the crossing of the Zone's border, although in the spirit of "action," looks rather strange: I believe that one could easily come up with half a dozen, or even ten, reasons and moments why the plan wouldn't work in real life. There are many other such things.
However, I also saw a good reinterpretation and further development of some moments, and, to be fair, the main character, in the spirit of Prince Myshkin, looks even more authentic than in the book.
I watched it at a speed of 1.25. It could have been even faster, but then the characters mumble their phrases too quickly, which spoils the impression. However, if you want such a slow and thoughtful contemplation... Although, no, the film is still too slow.
I have no idea how to rate this.
Tarkovsky/10, but that's a good rating.