I got Civ5 free with xcom from steam and have put a good few hours into it.
I also have read some reports that Civ4 users are not happy with some of the changes. Perhaps patches and/or Gods and Kings fixed some of these issues.
I missed civ4 for the most part so watched some lets plays to get the basics for Civ5. See here:-
http://www.youtube.com/user/quill18/...ow=grid&view=1