Finally got around to watching it on the DVR. It's a good thing it had a break in the middle, because I needed to watch it in two parts. It was interesting, and I tried not to hold the slow pace against it, since that was clearly a deliberate artistic choice. Still, some parts of it did seem arbitrarily prolonged. I don't get the point of that endless driving sequence they did early on.
The commentary on the Criterion disc discusses this. As I recall -- someone with the disc on hand can check -- the historians point out that the driving sequences were shot on location in Japan, an unusually expensive move for a Soviet film. Tarkovsky possibly kept them in because he wanted to show his backers where all the money went. Moreover, at the time, the city (Tokyo? Bad memory at work here.) was pretty futuristic.
Apparently, Tarkovsky cut out the score from those sequences at the last minute, which don't do them any favors. The historians on the commentary agree that it's the most problematic portion of the movie, and I agree.