Terrain Importer Features
Import Tile Sets of (theoretically) any size
By creating terrain prefabs out of your import files rather than generating the terrains in the scene (though both options are available, and can be used simultaneously), Unity's editor memory limitation can be circumvented.
Import one more more RGBA Splat Maps or a full coverage texture
The import settings for the splats are automatically configured prior to being applied to the generated Terrain. Any image format that can be converted by Unity to a Texture2D should be supported. You can also import a single texture and have it stretch across the entire terrain rather than importing a splatmap.
Import Height Maps under a variety of supported extensions
.raw, .r16, .r32, .ter, .tif/.tiff formats are all supported. Additional height map extensions may be added in the future.
Import sub regions of files
No need to import the full heightmap file; you can now import a sub region of the file instead, which in turn makes using super sized files possible (note, the sub region must be a power of two + 1 value and can not exceed the max allowable heightmap size of Unity).
Import into existing terrain
Import your heightmap or splatmap/terrain texture into an existing terrain, and choose which settings are overwritten on the existing terrain, and which are preserved. This functionality also allows you to use the tool as a mass terrain settings adjuster, as you can choose to not import a heightmap and just modify the settings of a large amount of terrain at once.
Import Settings are stored as Scriptable Object Assets
This allows for quick and easy re-importation. You can also create a base file which stores common import settings (such as the location where you import files are located on your machine), and then duplicate this file to generate new import settings that require less configuration.
Generate Prefabs for your terrains automatically
The tool takes care of everything.
Configure the Terrain Settings for all imported terrains
This is incredibly useful when importing tile sets (imagine having to change the Pixel Error/Detail Distance/Tree Distance/etc. for every terrain in the tile set after import!).