According the Wikipedia's unofficial list of successful Hail Mary passes in the NFL, there have only been 6 successful plays since 2010. If we make a wild assumption that roughly 25% of NFL games end with a Hail Mary attempt, that is an abysmal success rate.
Now that the Falcons have Devin Hester, they should ditch the Hail Mary as their last play desperation move. Instead, they should run a play designed specifically for Hester which we will call the Hairy Male. Rather than send 5 receivers downfield and heave a fifty yard pass into a prevent defense, the Falcons should do just the opposite. Assuming they are at least out to their own 40 yard line, the Falcons should line Hester up in the backfield. Then, as Ryan goes through his snap count, Hester should run backwards, and line up on the 5 or 10 yard line. Two other receivers or backs could also potentially move back there with him. Ryan should then take the snap, turn around and throw the ball 30 yards backward to Hester.
This would effectively create a surprise punt return for Hester, except the defense would not be prepared for it in terms of personnel, running lane discipline, etc. Meanwhile the two companion backs could set initial blocks and the offensive line can build a wall down field.
Granted, if the defense was prepared for a play like this and did not take the bait and run forward, it might be tough for Hester to find an open lane. However, the first time the Hairy Male is tried, the defense is likely to run up field toward Hester, making his job that much easier. Even if the defense does hang back, Hester and his blockers will be running with a 30 yard head of steam and might be able to break through regardless.
The only potential fly in the ointment is if there is a limit to how far behind the line of scrimmage a player can line up. I have not been able to find any such rule on the Internet, but that doesn't mean one doesn't exist.