I started off watching the show when it became available on Netflix, and it wasn't bad, but it definitely wasn't great either. I picked up the book, thinking it couldn't be any worse than the show and was rather surprised.
First, the whole genre itself was changed to adapt it for television. The book is pretty solidly a mystery with a dash of romance and fantasy elements, while the show focuses more on romance than anything else. Second, the main characters' powers are far more pronounced than they are in the show. This makes them far more interesting than in the show.
One thing that really surprised me was how badly dumbed down the book was during the adaptation process. There is so much to the plot based in history and mythology that is largely glossed over. Considering that other shows have focused on Norse mythology and been relatively successful, it wouldn't really be much of a stretch to say that the Norse mythology elements could easily have stayed in and been successful. It's almost like the networks assume anything geared toward women has to be vapid and shallow because our little minds can't handle anything smart.
In short, I recommend skipping the show and picking up this book instead. You'll enjoy yourself far more than binging the series.