Just like in real life, there's no absolute truth

High Essence is great for Conclaves, because they can get a building that provides +1 research and mana per Essence (that's apart from any spells!) They even get bonuses to Research, so yes, high Essence is great for Conclaves.

However, the warlords on this forum will point out that high Essence spots are great places to build troops, as you can use very powerful +1 bonus-per-Essence city enchantments to build elite units. So if you are planning on building some elite troops, it's better to build a Fortress on a high Essence spot and make use of all the Fortress buildings that improve units.

Towns only profit from Propaganda, for the extra Gildar. I wouldn't recommend building a Town on a high Essence tile.

Mind you, I am talking about 2, preferably 3 Essence. 1 Essence isn't that special, you can build whatever you feel like, depending on other factors.

Edit: Reading Phazon's post: yeah, never build a Conclave on a 0 essence tile. Sadly, the AI does it sometimes, but it is hardly ever useful, unless you really need the free +1 research per city level.

It is very useful to have at least one fortress on a two or three Essence site, so you can train troops with all the buff spells. Your capital is a good candidate for this. However I only ever really use one fortress to train my troops, so after I have my main fortress set up I would make any two or three Essence site a Conclave. There are several Conclave buildings which benefit from more Essence, Alchemy Lab springs to mind (gives extra production based on Essence). Handy for building the Towers and other late game buildings.

if you can't find decent spots with multiple essences, i'd still build a few conclaves on non essence tiles. most of their research comes from levels and the +% buildings (sage/cloister etc.), so they will be decent research centers even without the extra bonus from the alchemist. they won't generate a whole lot of mana, though. research is important and towns and forts are (almost) useless for research.

follow the "rule of thumb" to build conclaves on essence tiles preferably - if the land allows it, but don't ignore conclaves just because you can't find essence spots.

I would say try to make your capital on a 2+ essence spot and make it a fortress and after that use conclaves for all the cities that have decent essence.

Mana/Research/and Production per essence is a really good thing to have a lot of for Conclaves.  It really doesn't outweigh the Auras in usefulness, though.

I wouldn't say that a 0 essence Conclave is worthless, but I would much rather have a 3/3/3 conclave than a 6/3 one most of the time.