Helen Mirren and Christopher Guest do a fabulous job playing Tolstoy and his wife. James McAvoy is delicious as the young secretary (I swear he has one of the most expressive faces on the planet) and Kerry Condon does a very nice job as his love interest. The sex scenes between them are warmly lit and very sweet. Paul Giamatti, whom I don't particularly care for, does a perfectly adequate job, I guess - it's entirely possible that it's not his fault that his character is such a cliche. Also, when they show footage of the actual people in the credits, its astonishing how much he resembles the guy he's meant to be.
The problem is, first off, the movie drags. A lot. It clocks in at about two hours, and I was checking my watch a lot. This is mostly because the movie is kind of incoherent - it really can't decide what it wants to be about, and where the drama is in the story. So it kind of rambles from one point to another, stacking up half-developed ideas (there could be two young lovers! kind of like the old married couple! let's... do something with that! maybe later?) and never really generating any momentum.
It's a great pity, because the relationship between Mirren and Guest, ie the Tolstoys, is fascinating. One could in fact make an incredible movie about them. But this is not that movie.