The protagonist is Shevek, a physicist from Anarres, who has gone to Urras to work there & collaborate with their scientists - the first person to go from one world to the other since the settling of Anarres. The story starts in the middle, with this journey, and then in alternating chapters we both go forwards from there and also follow Shevek from birth to leaving Anarres. This juxtaposition of different threads of time not only highlights Shevek's growing disenchantment with each world in turn, but also is appropriate to Shevek's work on time & simultaneity (which hooks back into Le Guin's wider universe here with the potential practical application of an instantaneous communication device, the ansible).
The juxtaposition also forces the reader to compare the two societies. The nation that we see on Urras felt to me like a caricature of Victorian society - ultra-capitalist, wealth & status obsessed aristocracy pretending to be a meritocracy, oppressed and demonised poor living in slums. Anarres was settled by a group that were anarchist (with an almost religious fervour) - they invent a new language to avoid the capitalist assumptions of their old one, completely re-organise their social order etc. At first the society on Anarres sounds as if Le Guin is setting it up as a utopia, but as the story continues it becomes clear that while the ideal might have been a good one once people were putting it into practice it got twisted & didn't work quite as it promised to work. What it made me think of was the collapse of the various Communist countries in the late 80s & 90s (long after this book was published) & of Orwell's book "Animal Farm" - share & share alike is a fantastic idea but once you get real people involved you find the system changing so that some people have more of the power & more of the resources. And so it is on Anarres - theoretically you do the work you want, when you want, but practically speaking no-one actually refuses a posting even when that means it splits up families and it seemed like the mechanism for ordering postings is manipulable. So as, say, a composer if you weren't writing the sorts of music that were favoured by the in-group then somehow you'd always end up on postings that have nothing to do with music rather than ones that would let you teach or perform.
This is a "classic" of SF and has won several awards, but unfortunately I found it fairly hard to get into and it felt dated and preachy to me. Partly this is because it was written a long time ago - published in 1974, the year I was born. And styles change, and concerns change (always an issue if you're writing something with a message). I don't think this would have been written in the same way if it had been written following the collapse of the Communist countries, it felt like a product of the Cold War to me. One of the other things that had felt dated was the protagonist himself - making me think of early works by Asimov (tho I'm damned if I could back that feeling up with any specifics). But just now thinking about how to talk about that I had the realisation that the story Le Guin was telling needed that sort of driven not-quite-socially-ept scientist character. Shevek comes to see the flaws in the society of Anarres and to try and change it precisely because he is a man who consciously works out what the ethical behaviour is in any given situation, rather than blindly following what others do.
You'll notice I've not talked much about Urras so far - that's the bit I found very preachy. Anarres to me was both an interesting idea for a society, the result of a conscious attempt to create a fairer world and a world without government, and also interestingly flawed. Urras felt too much to me like Le Guin was rubbing the reader's nose in what our capitalist, sexist society would be like if taken to extremes. This impression wasn't helped by the fact that over the last 40 or so years the treatment of women by society has improved. So the attitudes of people on Urras about how women just weren't clever enough or capable enough of doing anything intellectual or societally useful came across as even more caricatured than I think they might've done to people reading the book in 1974. Maybe instead of thinking it was very preachy I should be filing this issue under "dated" and, like with the bits of Life on Mars that I watched, just feeling very pleased that I am an adult now rather than in the 70s.
I was also a little surprised by how shallow the characterisation of the women felt (this being Le Guin, who I associate with feminism). Shevek had to be male, there's no way she could have written the society of Urras the way she did and had a female protagonist - such a female scientist would not have been welcome on Urras to the extent that she would not have been invited. But the supporting women seemed fairly one-note to me. As an example - there's Shevek's mother, who represents the sort of person Shevek would be without his compassion. Having said that, tho, the male characters are also fairly one-note, so that's perhaps an unfair criticism. The story is really about Shevek (and very focussed on him, out of the individual people) and about the societies, not about the supporting cast.
So in summary - I think this was a good book and has many interesting things to say about politics and about people, much more than I think I got out of it on a single read through. But if I hadn't been reading it for a bookclub I wouldn't've finished it due to finding it dated and preachy, and I'm unlikely to read it again.