People seem to love Drow. So I don't know why the Duergar aren't used more to set-up Drow storylines. They occupy the same environments, participate in the same activities (genocide, slave trade, underground empire building, betrayal), have the same type of vision and vision affecting abilities (darkness vs. invisibility) and use the same type of weapons (crossbow at range, then melee if necessary).
However, Duergar aren't known for their poison usage, which I'm fine with, fewer rolls that the players have to deal with, fewer chances for radical swings in the flow of a battle. But that reduction in complexity is offset by the increased numbers complexity of the Duergar's enlarge person - being prepared with the numbers ahead of time negates this complexity.
The real difficulty with Duergars is that the vision rules have changed through the versions. Originally, low-light vision was the cover-all and it was in gray scale. So the Duergars skintones made for decent camo, as later editions came along, the separation of vision into low-light and darkvision changed how effective skintone camo was. The key for the players to understand and the GM/DM to remember is that low-light vision doubles normal vision ranges, whereas darkvision functions in complete darkness, but in gray scale.
Duergar present some complexity for parties to deal with. Their immunity to paralysis, phantasms and poisons negate some of the more colorful character builds while their stability counters characters who specialize in tripping, bull rushes and grappling.
One of the things that frustrates me the most is having a PC who's armor class has made them almost un-hittable by normal monsters. With the Duergar (and Drow), now you can play that same card against the PCs by giving them decent armor/shields/spells - just be wary of having the PCs use said equipment after they have defeated the Duergar.