My players are currently in the Caves of Hunger, having defeated Auril in her lair.
The Flameskulls lived up to their reputation of being extremely under-CR'd, dropping almost the entire party. This was, ironically, aided by the fact that two of them had become werepolarbears, as this otherwise-useful boon comes with a side order of vulnerability to fire.
Tekeli-li, on the other hand, did not work out at all. The adventure describes him as a recurring threat, attacking periodically and then turning to mist and retreating if he takes more than 20 hit points. This fails to take into account that a 9th-level party can easily get through all his hit points in a couple of turns - in this instance, the paladin and barbarian, going first, laid down over 100 hit points of damage, and that wasn't even using all the barbarian's attacks.
Since gnoll vampires apparently don't have a resting place they can retreat to, he was basically toast before he even got a turn, and the party sealed the deal with a stake through the heart and a sprinkling of holy water.
The relic that casts arcane eye on a person was also a bit of an oddity - it essentially allowed them to scout ahead a great deal, which meant the place lost a lot of its mystery. I ruled that they couldn't just scout the entire place dues to its size, so I gave them a choice of scouting either south or east, with the far south-east region being unrevealed whichever they chose. They chose east, and spent the rest of the session setting up ambushes and flanking attacks upon the undead in this area. The cleric was especially pleased, his Turn Undead obliterating all but two of the Shadows while they were still surprised.