3D World Studio (formerly known as Cartography Shop) is a lot like easy-peasy Worldcraft/Valve Hammer. This is by far the most straight-forward, robust editor available. Worth the money. Try the demo.
I can confirm that .3dws is very good.On the website there is a blitz tutorial/listing called advanced loading which I found very usefull as a framework for loading in and using levels in blitz. called advanced loading