If you pre-ordered the game and went to grab it yesterday, popping in your code for Juggernaut (who is awesome, by the way), then your day must have turned out fine.
If you did NOT pre-order the game and, thus, did not receive a free code for the exclusive Juggernaut character, you might have been disappointed to receive an error when trying to play with a friend. The error said something along the lines of, "your local data is incompatible with the host's data."
Fear not! Follow the external link below to grab the Juggernaut Playable Character Teaser from the marketplace--required to play with any friends that have the Juggernaut character downloaded.
The funny part is, both the redeemed code and the free teaser at 18.11 MB in size. Basically, everyone gets Juggernaut--but only those who pre-ordered can play him! I've found that if you join a match with someone who has the character in their party, you can enjoy the experience smashing through foes with Juggsy as well... so don't worry!The Juggernaut Playable Character Teaser is only available to Xbox LIVE Gold members.
Hope this helps.