I've seen parts one, seven, and eight. Of the three Part One is definitely the most moving. Somehow it just seemed so realistic. It really drove the point of human logic being fallible. In fact, I was so absorbed in the movie that immediately after watching it I almost felt like I had been the one involved in the spiritual crisis.
I really appreciate how Kieslowski managed to convey the essence of the first commandment (Thou shalt have no other God before me) without being preachy. In fact, religion hardly came into play at all. This gave the film a more universal appeal, by expressing themes that are relevant to those outside of the Christian religion.
The cinematography is impressive, especially this one scene where an ink blot appears from nowhere on the main character's work. It really sets the stage for the ending.
Of all Kieslowski's works that I have seen so far, this is easily the one I appreciate the most.