I don't know about you guys, but every time I've gone in there I've walked out with at least one 8th level scroll. The vast majority of trips I end up with no less than three 6+ level scrolls, a pile of lesser scrolls (lvls 1-3, usually about 3-6), meager amounts of coin and usually some general useless item or two of value. Things such as Lightstaves, varying mage belts/gloves and what-have-you.
I've never been disappointed with the loot and that's not even fighting Elketh ('cause those bloody shadow things are insane). Though, the majority of the worthwhile loot is placed at the end of the dungeon a bit hidden off to the side.
Far as I know, the two drop locations are as follows:
1) Second floor, the one that's filled with Flesh golems and a handful of Broken Ones and Zombie Golems. This are is three (floating) chests that contain generally crappy mage-stuffs (1-3 level scrolls)
2) 'Final' floor, up the stairs from the teleporter that leads to Elketh himself. There's something like 3 Wardrobes and a number of chests with insanely high OL/Disable DC's that are filled with mid-high level mage-stuffs like scrolls up to 8th level and Greater mage gloves