Im clicks away from buying this, but if i remember right, when i tried setting this game up ages ago i could never get proper mouse support working (like, looking around and moving properly without it constantly levelling out my view). Does this GOG version have proper mouse aim from the get go ?

Your best bet is using a modern port of the game, such as the EDuke32, which has a completely new OpenGL renderer; with the new renderer the mouse-aiming is smooth and works as if in any modern game.
If you want to use the DOS version, I think there are options for setting the vertical mouse speed, but it will still feel a bit of, since Duke's aiming always was a hack done with the renderer, which is why newly written renderers work better.

How do you map new buttons in duke?

I've just finished tweaking the keyboard config to resemble something like the modern WASD setup, it's quite doable actually and can be refined to your specific needs.
Go get it dude! :-D
You need to enter the setup mode, it's link can be found in your Duke Nukem folder that the GoG installer puts on the start menu. Short way around is to go in the instalation directory and double click setup.exe
there you've got the full keyboard and mouse configuration. If you get Eduke32 it becomes easier since it moves everything to the "options" menu ingame
Hope it helps!

I have finished with the setup and I am happy to see it works really well. It feels pretty much like a modern game. Gracias ;)

Can you guys post your configurations so I don't have to spend ten minutes fiddling with the setup?

This is really the best advice on the thread. Not only do you get the option of fully 3d rendered Duke, but it removes all the headaches of trying to get the controls and mouse set right.