Much like The Left Hand of Darkness, this is a novel of Big, Important Ideas. Shevek, the protagonist, is a genius physicist from the planet Anarres, a dull, arid rock that was settled as an experimental haven for a group of socialist anarchists 150 years before the events of the novel. Anarresti society is fascinating -- it's far from perfect, but it works. However, due to the planet's isolation and lack of resources to pursue his scientific theories, Shevek chooses to travel to the nearby planet Urras, a capitalistic world that greatly resembles our own Western society.
This is obviously an important sociopolitical commentary, but it's also a damn good story. I found it less dense and more readable than The Left Hand of Darkness, but no less thought-provoking. Le Guin is truly gifted at creating worlds so different than our own, with completely alien concepts and conventions, while her characters maintain more humanity than many of us.