I love using my 3D Space mouse and wish all 3D apps would support it, it's pretty life-changing. The only real question I would have with it in Terragen is around the way Terragen's progressive rendering updates. The space mouse works great with apps that have realtime viewport updates, which makes it feel very fluid, it would be interesting how it would work in applications that use progressive viewports, such as Clarisse and Terragen. I would love to have it as an option though. I guess it's truly a matter of the speed of your machine at that point.