Workflow was hugely important, we had to be able to transfer cameras (some tracked and others animated in maya) from maya to terragen.
We used a script in maya to convert animation into .chan files, and apply those back to the camera in terragen.
Worked well mostly, just the odd hiccup.
We also had a system of exporting .ter files and importing them (via a custom script) into maya. That was a huge help, as we could then animate cameras and export them back to terragen.
We only had dem data for one environment, the rest were created from scratch.
For these I got into the workflow of, once being happy with the prime landscape and major features, I'd bake that down to a .ter file, throw away my creative nodes and reimport the .ter as my base. This saved me loads of rendertime, and with so many long sequences to render, I had to save time where ever I could.
Chris