Going back into the "intro; yey or ney?" Discussion, I want to put in my two cents that something I found to be a good compromise was starting off the game a few weeks after the characters first met and simply frame it that they know each other well enough to work together but there's still a lot they have to learn about each other personally that can be fleshed out between, or even during, assignments. This lets you get the story moving at a decent pace and still gives players a lot of room to develop relationships and personal story arc.

Really, the key for anything is just keep the story moving. The games that get caught up on one or two things are the ones that burn.