Never played China - or any of the mods. I'm still playing Minors like the Teutonic Knights or Lenape, and having tons of fun. You are right - the replayability is not that high for any one Civ, and the Age of Exporation, Age of Reason and Grand Campaigns are very similar.
However, as an alternative to Civ it is very good - after over a year I still dip in from time to time and if I designed mods would do so a lot more. MP would be very interesting, although the real-time nature could make things quite frantic. When I play SP, I am usually in 1minute=4months, and the pause key is on half the time!