Reducing Zola's masterful but monstrously long novel to a movie is the problem that Claude Berri does not seem to have resolved. He sticks close to Zola's text, which means that we get lots of undeveloped snippets of what were very developed scenes in the novel. If you don't know the novel, this probably causes a certain sense of confusion. If you do know the novel, and it is well-known in France, you have the sense that you are just skimming the surface. I think that Berri would have done better to be less faithful to the novel, or at least less comprehensive in his adaptation of it.
That said, there are most certainly good things in this movie. Miou Miou delivers, in my opinion, the movie's best performance. No, she is not at all the earth mother that Zola's la Meheude is. But she acts with her face, saying far more with a facial gesture than many words would have said. In a movie that skims over a lot of material, that makes for very effective acting. Depardieu is sometimes very good - physically he is perfect for the part of le Maheu - sometimes he seems to deliver the lines without thinking about them. The actor who plays Souvarine is very striking.
The cinematography is nice, but does not convey a lot of what Zola emphasizes in the novel: the heat and lack of space in the mine tunnels, etc.
A good movie if you haven't read the novel; a disappointing one if you have.