I love a good con movie. The Grifters is an all time favorite. I'm a fan of House of Games, The Spanish Prisoner, The Sting, Catch Me if You Can. Seriously, I can't get enough of them.
Here's one key to a good con film: Even though we know the con is coming, the film should still have the capacity to surprise us. In other words, we may see the double-cross coming, but the triple-cross totally throws us for a loop. Focus does this reasonably well.