A pretty stark depiction of the human spirit and how quickly normal people can become dehumanised by their conditions, but balanced out by the characters played by McQueen and Hoffman (together with the chaacters that come into the story and help them out). It's not always the actions of the prisoners that are brutal either, the establishment - both guards and civilians - are also portrayed in a damming light. Brilliant acting from McQueen and Hoffman as they are able to pick up their characters and bring out their differing but complimentary personalities perfectly.
I'd agree with the comment that you wouldn't watch it much more often than once every five years as it's a long film and quite heavy going; but when you do you'll be drawn into it without fail.
Cinematically it's also very impressive with brilliant use of locations and atmospheric sound, including effective use of silence when some films may have shied away and felt that they needed music.