Don't freak out.
So many shows start out with great characters and good story, but 3 seasons in the characters have completely changed without reason and the story is just dragging. Chuck's characters are the same people they were from episode one, but they've grown and learned. They haven't changed who they are as people though. The writers also know how to let one story end and another one start without being repetitive.
It has its corniness and it clearly makes fun of government agencies, but it also has great humor, really awesome soundtrack, and a seemingly endless supply of beautiful women. When I started watching this show, I didn't think it would last, but it has become one of my favorites.