A sequel of sorts to Blindness, I would recommend you read that book first, as, while not vital to the plot's enjoyment, it certainly embellishes the sparsely illustrated back story and gives a clearer idea of just how bad things got four years previous to (and referenced throughout) this novel's chronology.
Saramago is a challenging writer; his insistence upon endless prose with little to no puntuation, and a refusal to give names to characters, let alone use the conventions of paragraphs and speech marks for dialogues, all add up to a slower, more arduous read - but perhaps a more detailed and careful one for that.
The story is as outlined, but I would say that I disagree with the other reviewer, insomuch that I felt the second half was plot driven as the first, rather it becomes a slightly different plot - an investigation. The story moves from macro-study to micro-study, but throughout, as with Blindness, concerns itself with the paranoia of power and the desperate, despicable methods invoked to maintain power and control.
Personally, I enjoyed Seeing more than Blindness, but that is mainly because Blindess is a far more harrowing read, not that Seeing is the better novel. Seeing shows us the folly and weakness of those in power - something one can examine with a degree of (powerless?) complacency; Blindness holds a bleached mirror up to each and every one of us and terrifies.