Steve McQueen's direction here is some of the finest you'll ever see. He's absolutely unsparing with the audience: "You're going to watch this, and you're going to see some things you'll wish you hadn't, but this is a thing that happened and because of that, you are not going to have a chance to avoid watching this."
Not even protagonist Solomon Northup (Chiwetel Ejiofor, finally gaining momentum for the case he initially made in Dirty Pretty Things for being one of the best actors of his generation) is let off the hook. When his twelve years come to their abrupt close, both McQueen and John Ridley's script make it perfectly clear, without belaboring the point, that Northup is exceedingly lucky to have escaped his hell. If he ever truly does.
The problem with everyone (including, I admit, me) harping on how intense and horrifying this picture is, is that non-cinephiles might think "this sounds depressing, I'm going to skip this." That's the thing: this isn't a depressing movie. It's the truth. And it's a master class in the effect a director can create by things like simply holding a shot without cutting away. As hard as it is to watch sometimes, the takeaway from 12 Years A Slave is, "I'm glad I saw that." It may be something better having seen than to sit through, but the latter is a prerequisite for achievement of the former.
Or, in short: you absolutely must see this picture if you have not yet.