Good find!
Just had a play with it -imported a terragen file and it output a nice clean mesh (I tried obj) - the only caveat is that the normals of the mesh were flipped, but that's easliy fixed in a 3d program.
Tried it to see if it would would cope with a huge .ter file - I made a .ter file with 4096 x 4096 points - took a while to load and unfortunately crashed when exporting to .obj (took about 20gb in my 24gb machine) - but that would be pushing it to its limits!
For a 3d viewer and format converter this is very useful even with out the Terragen conversion options.