It's also useful to note that this spot is perfect for using the Guild Battle Standard. http://www.wowhead.com/item=64402
It's a flag you can buy from your friendly guild vendor that increases XP gain by 15% for 15 min duration (10 min cooldown!) in a 100yard AoE (which covers the whole spot). If you're farming here and you aren't 90 yet, that +15% XP with 100% uptime is insanely good (;
I'd also like to put in my 2cents for the optimal route:
Talk to Ken-ken to start quest
Mount up, collect spawns from all 3 clusters
Bring them to the well in the centre (I plant my guild standard there to make it even more noticeable, it gets a little confusing after so much of going round and round)
Kill them all there, taking care to hug the well (this ensures you do not aggro any newly spawned ones)
Mount up, repeat.
I was able to do this on a Level 86~387 iLvl Blood DK with pretty safe HP levels (used Death Pact every 3-4 pulls).
Upon further testing, it seems like when you initially start the encounter, the max concurrent number of Essences is somewhere between 10 to 12. It's possible to increase this (I was able to make it go up to 15) but I'm not sure exactly how. I think it has something to do with killing 1 or 2 Essences and then killing their respawned versions together with the whole lot.