If you got the limited edition version of the game you could have got B2K free.
It comes with 4 maps, 10 new guns you can unlock, new vehicles, and new achievements/trophies. IMO the maps are only good for Conquest, Rush doesn't have jets. :p Weapons are worth it I think. The FAMAS, L96, and a few others are good guns. If you really like BF3 and think you'll be playing a good amount I would definitely get it.