The mods don't really add anything meaningful to the game. They fix some bugs, and add some new content, but there's no over-reaching mods that are *better* than the original game.

I did my first playthrough totally vanilla, and enjoyed it immensely. On later runs, I added the widescreen mod (just up to 800x600), a couple of the fixes from the Fixpack in the sticky, and a couple of the Unfinished Business things from the sticky. They added a small amount of depth to the game but honestly it was perfectly fine the way it was.

There are some games (BG1, IWD2, Oblivion) that benefit greatly from installing a metric ton of mods. Planescape is not one of them.