The DRM is a little cooky for this, in the sense yes you don't need an internet connection after activating, but on the flip side if you play offline you miss out on a bunch of stuff that is only available if you play online/with a connection.
This includes voting for world leaders with other players, which influences production. As well as voting for senate members which does something similar, theirs news briefs about world events and other things that are updated.
Its actually very cool, it feels like the game is still alive and not just shipped as is. I have a feeling like I'll be playing this more often then I expected because of all the new content added weekly and placing votes, ect.
Other things offered online are world event missions (right now pirates have the bomb) and daily/weekly? missions.
So yes you can play it offline, but you miss out on a lot of content that's being added weekly not only by the developers but the community as well.
I suppose some will argue that they should these updated world events without it being online but...thats like playing multiplayer without any other players...and its not like your paying extra for the daily missions and world events.
So I guess let me put it this way, if you don't have a stable internet connection you can play it, but if you do it add a deep extra layer of live beyond the campaign and random map generator.
I don't know what the point of the DRM was anyway. The online content can't be accessed unless you're connected to the server. It does nothing to deter pirates either since a crack is already available to play this offline.