Wolv and RD use QuArK exclusively, and they have each released damn good maps.
The thing about QuArK is that it has loads of awesome stuff in there for the "experts", but the user interface (especially the 3D view) sucks. A partial list of awesome QuArK things includes:
Path Duplicators: draw a curve in 3D, and then choose an object to be extruded along the path. You can make complicated knots and such in minutes.
Diggers: like carving, but "safe". You could make entire maps UT style with these if you wanted. Also, you can specify that the digger cuts into some objects, but not others.
Tree view: coolest thing in QuArK. Everything in the map is automatically organized into a hirearchy, and you can further group things if you wish. Sort of like visgroups on crack.
Instance Duplicators: Takes a single object, and "copies" it, except whenever you modify the origional object, the copy is automatically changed in the same way.
Coumpass: You can "turn" the views so that you are no longer stuck working on only the default coordinate axes.
Suppose you want a smoothly curving 3D tunnel system (something that is extremely difficult to make in Hammre). In QuArK its easy:
1: Draw a path in 3D that you want the tunnel to follow.
2: Make a cylinder.
3: Use the path_duplicator to extrude the cylinder along the whole length of the path.
4: Make a box surrounding the "tunnel system".
5: In the Tree View, add the path_duplicator to a new Digger, and move the box to the correct hirearchy so that only it will get "dug".
Done!
http://maze5.net