Adapting a manga from a by far famous novel is no easy task, while the mangaka for the most part translates the story well, there appears to be something missing..
Storywise, it's extremely faithful to the novels, however, many important plot-points from the novel appears to be lost. And for readers such as myself, the only thing aiding in understanding the sometimes jumped narrative is prior knowledge gained from the anime series or the novels themselves. Sometimes sudden jumps occur throughout the novel, as if the mangaka Tsugano simply wanted to 'tick' off the best parts of the series.
Artwork is a hit or miss, containing much less detail when compared to the illustrations shown in the Light Novels. Panels are sometimes cluttered, and memorable characters can sometimes be portrayed 'off-model'.
Overall, for anyone not interested in reading the light novels, this provides a decent, sometimes confusing alternative. I would still recommend the anime series or the novels themselves if you prefer a much easier way to understand the series.