Hammers + Set in Stone = 150+ Production! I say set Hammers to +1 Materials and keep Stone as is. That would then require research to get OPed.

I used stinking mud quite a bit in my last game; mostly for end quests; where I wanted to immobilize large number of opponents while my other csaters charged fireballs; worked like a charm - and made those quests very easy.

Earth tier 1:

Enchanted hammers - use a lot

Lower land/raise land - sometimes

Nature's cloak - sometimes

 

Earth tier 2:

Stoneskin - sometimes

Tremor - sometimes

 

Earth tier 3:

Set in stone - never

Shockwave - sometimes

 

Earth tier 4:

Fracture - never

Summon earth elemental  - never

Destroy land - never

 

Earth tier 5:

Birth of summer/bloom of twilight - never

Earthquake - sometimes

Giant form - never

Diamondskin - never

 

Combo spells:

Syphon strength - never

Cloak of thorns - never

Stinking mud - never

Nature's call - once

Hmmm looks like destroy land and bloom/birth have been removed or are hidden away in some quests - perhaps they're exclusive to the campaign?

Earth tier 1:

Enchanted hammers: A great boost in the early game, and worth continuing throughout.

Lower land/raise land: I rarely use this, though as an old SMAC player, I can understand the strategic value of terraforming.  It would certainly be a lot more final to lower all the land around the edges of a rival's territory, instead of setting up outposts for them to ignore.

Nature's cloak: Excellent boost.  Very useful even in the early game against air shrills, and keeps helping throughout.

 

Earth tier 2:

Stoneskin: Another must.  Because no matter how good at dodging your melee types are, they will always come up against something that's simply more accurate.

Tremor: At best, a stopgap measure to halt roaming monsters while you desperately try to get an army there.  I prefer Pillar of Fire.

 

Earth tier 3:

Set in stone: Don't underestimate this on a fortress.  As long as you've got a few towns churning out gold, you'll have a fast-producing army machine at your disposal.

Shockwave: Sure, I should drop my mage champion/sovereign right next to a group of attackers in order to use this spell.  Riiiight.  Of course, if they can't be hit...but then, there are other mass effect that don't require the setup.

Earth tier 4:

Fracture: Not bad.  Works well against some creatures that have high resistance to fire, for instance.

Summon earth elemental: Nice.  Haste one of these kiddies, and let 'em rip.

 

Earth tier 5:

Earthquake: I'd rather just take over somebody's city, than half destroy it.  And you can't do this at a distance--you have to see the place with an army to level it.

Giant form: Worth getting all the way up the Earth tree for this...

Diamondskin: ...and this.  These are boss killers.

 
Combo spells:

Syphon strength: Almost never play the Death book, so I can't comment.

Cloak of thorns: Not a primary defense, but useful on top of everything else--if only because it's fun watching some of the damage bounce back to the caster.

Stinking mud: Great spell.  Nothing but nothing beats watching those stupid mites slowed to a crawl.

Nature's call: Useless.  You've gone up two spell trees partway, and you're rewarded with a spell that summons something which makes a wolf look dangerous.

 
Just my two cents.

needs more damage doer type spells and summonses. 

It is random with a 75% chance to show up. The first thing I mod when I get a new version of the vanilla game. ^^

The graphics of the Stinking Mud spell look more like an insect swarm to me. Maybe we could get something that really looks like muddy ground. And as an aside, some kind of insect swarm spell might be cool too.