I use Terragen a bit for backgrounds in my Bryce art. Some of the artwork people have done just in Terragen is amazing though. It takes a bit of work, but I found Terragen very easy to use. I also tried out Mojoworld which can create some great landscapes, but the render times are crazy. Sometimes you have to turn off shadows and smog if you want to get your render done in a day. It's also took me a while to learn how to use the program and make planets from scratch. I still don't exactly know what I'm doing and most of the cool scenes I make are completely by accident. Mojoworld is worth checking out though. You are able to import objects too which is nice.