Yes, Offline is coming as a free download with Update 10 to all SimCity players. When we launch it, all of your previously downloaded content will be available to you anytime, anywhere, without the need for an internet connection. We are in the late phases of wrapping up its development and while we want to get it into your hands as soon as possible, our priority is to make sure that it’s as polished as possible before we release it. So, until then… testing, testing and more testing. As one of the final steps, we’re putting Offline into the hands of some of our most hardcore players, the DevTesters. This group of volunteers is going to put Offline through its paces before we release it.
I still recall them saying "SimCity Offline is NOT possible" and that the game requires too many calculations handled by the EA servers for it to not function if it were to be offline.
Bringing the game Offline means big things for our wonderful community of Modders. They can now make modifications to the game and its components without compromising the integrity of the Online game. Modding is a big part of our studio’s legacy and we’re excited to see what you guys create. Check out this thread to learn about Oppie85’s Central Train Station, which you can put into your game right now, and if you’re interested in making your own content take a look at the Modding Policy. To get you started, we will be rolling out a series of tutorials from the studio that surfaces how we’ve created some of the content that you’ve seen so far in hopes of inspiring your creativity.
But now with this announcement, perhaps I'll finally consider picking up the game now. It was my biggest gripe.