I am the only person AFAIK on Ecomodder with a C-Max. Allow me to share my experience and my personal feelings on the matter here;
At this point, I have 3,400 miles on my car. The Lifetime average MPG is currently 46.8mpg. That includes 100 miles of test drivers beating the car before I bought it, and the first 1,000 miles or so of the car learning my driving style and only returning about 43 mpg. So, the last 2,400 miles have been around 50mpg. I have seen numerous 120 mile days above 52mpg.
Now, why the mileage reports showing low averages? Well, my theory is multifaceted. Here are my thoughts on it;
First off, the C-Max has a combined output of 188hp. It just plain moves out when you get on it. Also, it has low profile, wide tires with a low stance. It handles very well for what it is. These two things make it an exercise in personal restraint to try for decent mpg. Most people that drive it enjoy it so much they tend to get on the gas and drive it hard. Ford made this car as a drivers car, not just an exercise in extreme efficiency.
Second, (at this is confirmed on the C-Max forum I frequent) the car requires a good 2,000 miles or so for the computer to memorize your driving style and compensate along with the normal break-in period. The C-Max is so new, most are not even over that break-in/learning phase yet.
Anyway, I am absolutely in love with this car!
I cannot wait to get a full pan and some other hyper-miling mods installed to see what it is really capable of.