I read somewhere that the videographer shoots the signals while talking into the camera telling what's actually happening with the opponents D on the field. This can obviously be looked at fairly quickly and some basic info deduced.
The Globe has an article that gives some insight into the advantage:
http://www.boston.com/sports/football/patriots/articles/2007/09/12/in_nfl_stealers_have_edge/
The key advantage seems to be that it could help you in a critical play or two if you succeed in deciphering the signals and communicating that to the QB.
From the article:
Mark Whipple, the former University of Massachusetts head coach and Pittsburgh Steelers quarterbacks coach. "'It always comes to one play, the fourth quarter, and if you can make that call, at that time, and you know what they're doing, it's a great advantage.'
...
"One NFL assistant explained that the offense already has a built-in advantage because the quarterback has a communication device in his helmet that allows him to hear an assistant coach while in the huddle. If the offensive team identifies the defensive personnel on the field - and decodes the signal relayed to those defenders - it's possible the quarterback could be told what to expect."
...
"'If you know the coverages for the passing game and when a defense is going to come with a blitz, you can have the counter for those moves ready and it sure makes it a lot easier,'" Whipple said.