I experienced the same behaviors described in this thread. I submitted a support request and the behavior was confirmed.
After several exchanges with support the issue is being escalated. Here is the final reply I received:
I am sorry that the DEM files created in Civil 3D do not work correctly when importing to ESRI products. Having spent the better part of the last 12 years relying on software to do my work, I know how frustrating it is when something does not turn out as expected.
Don't know how helpful this post will be, considering that it's already been escalated up to the dev team, but I've never had any problems like this when exporting from C3D to ArcGis. I use the metric system all the time; maybe the issue is related with the use of feet? And like mentioned above, I also use the geotif format. I also have issues with the other formats that can be used.
I have been trying to solve this same riddle for quite some time. I found a web page that provided an answer at http://thecadgeek.com/blog/2008/06/creating-a-dem-
To be able to go from C3d to a DEM usable outside of C3d would be the best.
What happens when you run the _SURFACEEXPORTTODEM command?
You should have both .DEM and Geotiff output: try 'em, then load in Microdem, for instance.