What works
Everything on the single-player campaign in brief testing.

What does not
The toolset. Prerendered movies and the various corporate credit screens at the start either crash the game or only play audio, so better off disabled.

What was not tested
Multiplayer. The two main expansion packs and the other premium modules the Diamond version ships with (except Kingmaker, briefly tested), but I'd be surprised if they had any problems.

Additional Comments

Purchased from Good Old Games. GOG.com modifies its games to improve compatibility with modern Windows systems, so that may affect results with Wine.

Silver rating chosen because of the toolset and movie failures, but single player works flawlessly as far as I can tell with the following settings. I see 15-30 fps on a late 2008 MacBook Pro using either the integrated Nvidia 9400M and discrete 9600M GPUs.

With Wineskin I suggest using Screen Options|Override, Rootless, Virtual Desktop at 800x600 (this only affects the initial launcher). Edit nwn.ini to say under [Display Options]

FullScreen=0
AllowWindowedMode=1
SafeMovie=1
Disable Movies=1

This will disable the aforementioned non-working credits, movies, and the full screen mode; by turning full screen off, Command-Tab, Spaces, and Exposé still work. From the nwn.exe launcher, run Configure, and select a resolution; this controls window size for the game itself. Enjoy!

PS - Although you should have no reason to ever do this, do not press Command-Alt-A to activate X11's built-in full screen mode; this badly confuses the game. Either push Command-Q to quit, or Command-Alt-Escape to force kill the app.)