OK folks. I have never played a campaign. I only play sandbox games. This includes other games such as GalCiv 2, the Age of Empires series, Heroes of Might and magic, etc.. What am I missing? Are their some elements that don't make it to the sandbox? Will it make me a better player? I have to confess..,.I fell I'm a mediocre player at best and would get destroyed were I to play online, lol.
Thanks to any who respond.
I think the campaigns basically help you get a feel for the game as well as helping reinforce the lore. For instance, the story behind GalCiv is almost as interesting as the game. If it were a book or series of books, I'd download them.
Hey, maybe that's something for Brad and co. to mull over. I'll keep an eye out at Amazon. 
As far as playing online, I doubt any of us are super good at it. That's why we can do teams and play against the AI (or other teams). We can also pass strategies back and forth via skype and really get confused.
