The Spirit Realm

Assuming a Specialize Alteration skill of 255, a 15% mod should save you 1.9% of the _base_ cost of the spell. This means the more mana preservation and mana regen you have, the greater the relative benefit you'll see.

Some calculations as a druid attempting to maximize the length of time that I can heal on an "endurance" fight: (Note that I have not experimentally verified these numbers; these are based on my understanding of the calculations behind mana preservation.)

As a druid I use Ancient Chlorobon as my primary heal. I have SCM3 (10%) and a 25% mana preservation focus for that spell (average of 13%). Specialize Alteration 255 = 12.75%; with Specialize Alteration +15%, that becomes Specialize Alteration 293 = 14.65%. Assume that I'm using a Benefit Awareness potion for 5% mana preservation on top of that.
Suppose I have 178 mana/tick regen normally, and that I have up for 18 ticks out of every 100, which provides an extra 18% mana preservation (in place of the 5% from Benefit Awareness) and 50 mana/tick regen. With Mastery of the Past fizzles do not apply.

(Edit: I forgot to factor in Gift of Mana. This can be approximated by multiplying the final mana cost by 0.9.)

Suppose I cast Ancient Chlorobon once per tick. The base cost of Ancient Chlorobon is 723 mana.
* Without Specialize Alteration +15%, without Form of the White Wolf: Average mana preservation is 40.75%, for a mana cost of 428.4 (385.5 after GoM; net loss of 207.5 mana/tick).
* Without Specialize Alteration +15%, with Form of the White Wolf: Average mana preservation is 53.75%, for a mana cost of 334.4 (300.9 after GoM; net loss of 72.9 mana/tick).
Weighted average: -183.3 mana/tick.
* With Specialize Alteration +15%, without Form of the White Wolf: Average mana preservation is 42.65%, for a mana cost of 414.6 (373.2 after GoM; net loss of 195.2 mana/tick).
* With Specialize Alteration +15%, with Form of the White Wolf: Average mana preservation is 55.65%, for a mana cost of 320.7 (288.6 after GoM; net loss of 60.6 mana/tick).
Weighted average: -171.0 mana/tick.

In other words, under optimal conditions using Ancient Chlorobon once per tick, I can last 7.2% longer just by adding a Specialize Alteration +15% mod.
"Real world" results obviously won't be as impressive, but it's still nothing to sneeze at.

all i have, atm, is an anguish helm for this, but truly believe that it helps me with mana conservation. i am now going to try to get my friends to try the ldon event to get those augs for all the alteration casters. my reason for this is that the change in mana use can be overcome by a far superior helm, or a really nice ear that lacks that mod. take the tier three helm from SoF, it has 4 clairvoyance and more mana and hp (need hp to canni from, ahem) unaugged than my anguish helm WITH an anguish aug, etc.
my point is that while i really love the mod, i won't let it cloud my judgement when i can get a core for my helm. just a mod of any kind can be made not so great when the mod is all that will be better than a far superior drop, imo.