Well, I have just finished using env3d in my first year class. Feedbacks from students are very positive. I have also learned a lot about designing a good API and have been making changes to env3d through each assignment. Lots of work but I think the results are worth it. Here are some students videos on youtube.

I'm now working on using env3d in the follow-up course which teaches inheritance and software design. A few new improvements are already in CVS, the most notable being a new EnvCanvas class for use in GUI applications, a new EnvObject base class, and a new EnvObjectInterface. These new classes will (hopefully) be used to illustrate the usefulness of an inheritance hierarchy.