A Decent Idea
The Cerberus Network is an interesting idea. What is essentially a pipeline for free content (assuming you purchased an original copy of Mass Effect 2), turns out to have some nice perks.
From the outset, C.Net contained "Zaeed - The Price of Revenge" and a mission to visit the crash site of the original Normandy. While the latter isn't terribly interesting (essentially a short scavenge mission with no enemies), "The Price of Revenge" gives players access to a new team mate Zaeed Messani, his special power Inferno Grenade, and the M-451 Firestorm (a flamethrower Heavy Weapon).
Since its release, C.Net has also provided several other free offerings - The Arc Projector Heavy Weapon (great tazer that jumps to multiple enemies), Cerberus Weapon/Armor pack (armor isn't bad, the longer range shotgun is awesome), and the Firewalker missions ( 5 quick driving missions in a slick hovertank).
C.Net also provides in-game access to paid content but those items can also be obtained via the marketplace.
The issues C.Net has aren't deal breakers but they don't do the whole package any favors. While its cool to have free content available to people (who purchased the game in such a way that the publisher actually sees revenue), most of the content thus far is forgettable at best. Nothing will provide that little edge needed during an Insanity play through and at lower levels its just more Mass Effect 2 (nothing remarkable). C.Net also suffers from some technical issues, most notably that I needed to leave the game for the download of any content to happen.
All in all, if you bought the game new it's a great little bonus. If you didn't but want to see everything ME:2 has to offer, you can buy it for $14.99. Otherwise, it won't hurt to not have it and the content it provides won't make or break your play through.