Is Terragen Good? Or is There Something Better?
I think Terragen is a nice application, which needs a little while to master. The next generation of Terragen will be even better.
Just download the free version and play with it to see what you can do with it (don’t forget to read the documentation/tutorials!). There is even a script to import the terrains from Terragen into Blender.
Isn’t Terragen only free during a trial period?
No, I believe it is free for “personal, non-commercial” use with no time limit.
However, there is a limit to the size of the terrain and the rendered image.