It appears as though Chris Evans' Captain American has found his Bucky. Sebastian Stan will take on the role of Cap's best friend and sidekick. It will be interesting to see how big a role this becomes. Bucky is most often seen alongside the superhero during WWII as a sort of hype-man, but he later takes up the villainous mantle of Winter Soldier. Eventually, Bucky eschews his bad guy persona, even filling Captain America's boots for a time after his infamous assassination.
As to how Stan will perform in the role, I have no idea. He looks like a good choice, but I've never seen the guy act. He had a prominent role in the short-lived NBC drama "Kings" and regularly appears on the CW's "Gossip Girl". This year he can be seen on screen in HOT TUB TIME MACHINE and Aronofsky's upcoming thriller BLACK SWAN.