| Well Maplet is not a world editor, it's rather a CSG (constructive solid geometry) mesh editor with an embeded Lightmapper.|
Currently I am using several tools to create levels:
Maplet for buildings or most parts of buildings.
Decorator to apply nice textures to Maplet exported buildings.
Several utilities like the Simple Tree Creator for Trees, plants and things.
A modified version of the FLE project that allows to have holes in the terrain for terrain-like mesh-terrains. This one is using the slope texture functions to make good looking terrains with ease.
a new, unreleased version of slimshady that allows to merge all these meshes. It also allows to alter the brush properties to create nice prefabs. Omni lights can be placed. Then the whole thing will be exported, with all brushes intact (alpha, masks etc.) including giles compatible light nodes as a B3D file.
In Giles I add sunlight and a few tweaks, then it will be lightmapped in giles.
The best way to edit B3D meshes (eg exported by Maplet) is to load it in giles, then export as 3ds, then edit in any common mesh editor that handles 3ds correctly, then save it as 3ds again and reimport in giles. You only have to make sure the texture filenames are no longer than 12 chars (8.3 filenames) to make sure 3ds will keep them intact. also, this will not export/import the lightmap, so you should do this before lightmapping.
As a last step I would use a patching utility that will replace the filenames of masked textures with their DDS coutnerparts.
This is a pretty complicated art path, but it was for free to me (since I already purchased giles).
At the other hand, a tool like 3d world studio by halo may be a good and compact solution, although you have to spend a few bucks.