I've enjoyed using TerraRay for some months. It's in the tradition of terrain creation apps like Bryce, but at a much lower price. I don't think it is quite as photo realistic as some other 3D landscape programs, but it is continuously upgrading features and quality.
TerraRay 6 is just out, and rendering speeds have been increased by 25 to 50%. This latest version can use 64 bit processors to speed up processing. Even better, the US $19.99 Mac app is available through the app store for $9.99 through December 7. It's a fraction of the cost of other similar programs.
I've tried the latest version and can confirm the speed increases. This new version also improves rendering of clouds, grass, rocks and adds some post processing effects to make your creations even more realistic.
I find TerraRay to be a good starter to mid-range renderer. If you were thinking of picking it up, this sale is even further incentive.
Check the gallery below for some samples.