So, firstly, I must say that I read the book a week ago and I still have a fresh memory of it. Today I watched this movie, and I would say that you better have to read the book first, so that the movie don't spoil it, in case you read it. And you really should to.
The idea behind the movie is really great, and unfortunately, it fails to pass the full meaning very well. However, you should keep in mind that it is a movie, and the book wasn't made for being one. That happens because of the great emotional presentation and the unique character and thoughts each person has in the book.
So, watching this movie, you lose a part of Dostoevsky's masterpiece. What is more, many details are missing that really give life and making the novel feel real. There are a few differences with the book as well, insignificant however.
"So must I assume this movie is garbage?" Of course not. The actors are great in the majority of cases, and the plot is very solid and good enough conveyed. Nothing great at effects or music (not more than 5 soundtracks, think so). But it's a decent visual representation of the epic novel. Read it and watch the movie as I said, you won't regret it.
PS: Keep in mind that they can be both depressing enough, but Very Good at their class (especially the novel).