If you feel limited by what Terragen can do with terrain, in particular *heightfield* terrain, then getting something *like* World Machine might be worth trying. Personally I would not recommend investing in World Machine at this time, though I am sad to say it.
Although Stephen is back to developing it at a reasonable pace, there are numerous remaining concerns, not least of which is the largely unexplained year+ absence in the recent past of World Machine's development. Who knows if that will happen again. But I would argue more importantly the fact that World Machine now has a fairly outdated user interface and set of functionality, it is a bit clunky by comparison to newer tools.
And that leads to perhaps the most important point, which is that for a long time there were no real competitors that were at the same level of quality of output and functionality as World Machine. But there are several tools introduced in the last couple of years and in rapid development that either already equal World Machine, or soon will, and have newer UIs, and faster terrain generation and editing. These include World Creator (GPU-driven, very fast response) and Gaea (from Quadspinner/Dax Pandhi who made plugins for World Machine that already improved on it). Both are in my opinion better options to look at now and be more "future proof". If World Machine is going to catch up and remain relevant it's going to take a lot of new development and probably a total overhaul of the terrain preview system, node network, etc.
That being said, World Machine has a free version you can try out and see if you like. World Creator doesn't, as far as I know, though they've said they will at some point (after arguing against it for a long time). Gaea is supposed to have an "open beta" some time soon-ish.
- Oshyan
