So I just started a single player campaign in ME2 and I'm about 6 hours into it.  I traded for it on Goozex, so I didn't get a new Cerberus code.  I didn't realize when I started this play through that the Cerberus Network granted access to additional content in the single player experience.  I thought it was just a gateway to DLC.  Given that I wasn't sure I wanted to buy any DLC for the game at the time, I didn't get Cerberus right away as 1200 MS points is pretty steep just for the right to purchase more content.  But now I'm reading that new weapons and a special character opens up if you have Cerberus even in single player.  Can you purchase Cerberus after you start your campaign?  Does the game register that you have it after the fact?  Has anyone done this and am I missing much by not getting it (or making it more difficult for me to play through the game without the hidden content?) .  Thanks.

Not sure about your question regarding when you buy and how it effects an in progress play through but do not buy the code. The Cerberus stuff is awful, all of it is worthless your better off buying some of the other DLC or waiting until the new DLC comes out.

the Cerberus code basically gives you dlc missions, a couple of new weapons, and a new teammate. As ShaneDev said they are terrible and not worth it. It does not make the game harder or easier(well the new gun might). However, if you decide to buy them you can play them anytime just like the dlc. Just buy shadow broker instead.

Though I agree with others that none of the free Cerberus DLC is amazing/must have, I would argue that it is still worth getting them if you really enjoy the core game. To me Mass Effect 2 was just incredible, and the extra content was very much worthwhile given the game is so good.

I really wish EA would stop with this "game registration" bullshit.

They arn't going to. Everyone has seen that it works so we're going to be seeing it more and more. They really want people to buy new product and stick it to Gamestop in a round about way.
 
As for the Cerberus Network I personally quite like Zaeed and some of those free weapons and armor have their uses. The Normandy Crash Site is also a touching moment.