So has anyone else played the co-op challenges with anyone? I did a few while waiting on a friend and got my 1star achievement but when my friend joined me and we replayed it he got nothing. Anyone else experience this?

There might be achievement bugs, or I haven't finished something properly yet, cause I've got the two achievs when you finish each campaign but not the one that's for finishing both campaigns, so it must be labelled wrong or I don't get it. "The story has just begun" must be mislabelled, you must need to finish all the "what if" missions too.
Here's a screeny, mine are in red.
http://imageshack.us/photo/my-images/38/willl.jpg/

I believe for that you also have to do the what if missions as well.

I have all the achievements. You can only get the 3 star achievements while playing solo.

Now i have all 4 achievs: for both campaigns not including What if, and for both camps. including What if, and still no "Story has just begun" achiev.