Yuan-ti are some of my favorite villains. I do like using Drow as villains, but I liked using yuan-ti as mysterious, complex villains. I am actually a bit miffed that they put them in Icewind Dale, though, because that made some players more familiar with them.
And yuan-ti are nasty. I ran one 3e campaign that had the players investigating a slaver ring that led back to the yuan ti. The yuan ti were paying top dollar for slaves with classes and levels (so they could convert them to tainted ones) and took over the bandit kindoms with their considerable capabilities.
The party at the time was quite the wrecking crew, but were a little overconfident when they finally came face to face with the yuan-ti. For one, Yuan Ti are very intelligent and I play them that way. Two, they have some really nifty abilities and feats. IMC, they tend to try and bog their opponents down in entangles and use expertise to dull the effects of the party's uber-warriors.
In the last game of that campaign, the party assaulted the yuan-ti lair, hoping to lay in wait for a caravan full of slaves that were arriving. The entered the lair and managed to kill some wandering yuan-ti, and set up an ambush in the entry way. However, they used spells like sound burst that announced their presence. The other yuan ti caught wind what was going on, and given their intelligence, knew better than to jump into the fray. They left the lair by another exit, warned and regrouped with the caravan yuan-ti and set up an ambush of their own.
The party set their halfling rogue to hide outside and spot the yuan ti. A yuan-ti ranger with camoflage on spotted her first, and started peppering her with arrows. She didn't get the clue that she was spotted and outclassed, and failed to run. She died before she could warn the others that the gig was up. The resulting ambush took out half the party. It was glorious.
JoeGKushner said:
One of my biggest problems with Yuan-Ti was that you pretty much had to customize each and every one to make a useful encounter.
I really don't agree. I threw in classed yuan ti in many encounters (trivial to do with Jamis' NPC generator), but the majority of Yuan-Ti I used were straight out of the book. Their set of abilities combining stealth, defence, magical/psionic and conventional combat makes them really versatile.