I like TE->Render->Save Image, then use the saved image as the elevation map for the terrain approach. Mainly because I suck at digital painting. I also use photographic textures in this way.
Works particularily well when you use the same image for both the elevation map and the texture of the terrain. Sometimes editing the texture part with a image editor is advisable. You can also optimize a reflection, specular, or whatever else map you want off of the base image.

Friends don't let friends use booleans.