In my opinion this is Christopher Nolan's greatest piece of work and one of the best movies of the last 20 years.
It's smart, twisty and comes before Nolan got embroiled in pouring so much spectacle at the screen it ends up being a distraction from the story. There's a tiny bit too much exposition in the final scenes but given the way it plays out a recap for the finale was an inevitability, lest people complain it's too complicated.
This is a movie I caught randomly with no foreknowledge when it was first released and then evangelised about to other people, 10/10.