IIRC, you need an internet connection period, just for the first-time install verification. Beyond that, the boxed version will never dial home afterward. The choice to update or not is up to the user who buys the boxed version. As I understand it, with the digital version, it will automatically patch and update just like Steam does (i.e. without your express consent unless you change your options.)
You'll have to let us know how it plays, and if it has any long-term replayability.
I can say, right now, your mileage will vary. I've played two games of Gal Civ 2. One epic, 20 hour game and one 4 hour game to get my feet wet.
Good as it was, I didn't have any compulsion to go back and try a new race combination, or a different overall strategy. And like Master of Magic, and Heroes of Might and Magic, I don't have any real compulsion to play those more than once either. (Well, HoMM anyways. MoM is almost worth it because spell use is so restricted, and there are two open planes to play on.)
So maybe that's me, or maybe that's the genre. But Elemental has a real SP campaign, apparently, in addition to the sand box. And all the mod features. (The map making particularly will eat up a ton of my time.) So, for me, I know it's going to be good for more than one sand box game where I see all the features, bend over the easy difficulty AI and call it good. Others that don't enjoy modding may not find it so. It all comes down to the RPG mechanics, and how much you can cram into one single game.
(As an aside, this has always been my issue with 4x games. When you dial the difficulty down to easy, you usually can see/do/experience every facet of the game, just with more or less bonuses here and there. That doesn't make for a lot of replayability in my book; I don't need a second play through just to see what a Fireball is like at +100% mana, or what city population growth is like at -20%, ect...)
I'm hoping between Races, Factions and Map features that get generated, Elemental offers a lot more real choices than say, MoM or Gal Civ. And therefore more replayability.