Iron Will

How does it advance in gem levels?

Is the cast speed penalty reduced or what happen?

In general it would be nice if not just the quality bonus is shown but also the max level of the gem.

I had it before last legacy migration, would kill to get another one. Its a very nice damage boost. Goes great with Molten Shell, obviously, but also opens up interesting build possibilities for templars and marauders.

Very nice boost to Fire Trap.

Is there any particular reason this doesn't exist in the passive skill tree? (See Iron Grip support and Iron Grip Keystone) Is it just a point of balance to require that one extra support slot to benefit from strength for spells?
Also, is this a quest gem?

This skill is amazing - I repeat AMAZING - on a Tempest Shield/Molten Shell build (Thank you build of the week 6).
Due to Tempest shield only having one cast animation its "slower cast" is no real penalty at all, same with Molten Shell. However, the big damage boost it provides is more than adequate to warrant this being a required gem in that build.

I agree with Zedkiel; amazing when combined with Tempest/Molten/Firetrap/etc.

Any skill where cast rate is not an issue makes great use of this gem.

The cast rate is not a bad downside. I just pair it with faster casting. After a few levels, the reduced cast speed is negated.

If you're using this support gem on a skill that has a cast time, it feels like it is best to use it on a single target skill like fireball. I was changing up my gem set up and noticed I was more effectively dealing with groups and rares by having my iron will supporting just my fireball and leaving firestorm to just benefit from faster casting. I have almost 400 strength and the decent spell damage increase +about +80%) was not felt much on firestorm, but made my fireball dps jump up through the roof.

Best thing about this support is that it has no Mana cost to it, which makes it perfect for my planned Flame Totem skill stacked with GMP, Increased Area of Effect, and Iron Will.

I'm currently running a fire-based Marauder focusing on Iron Will, and it's working fairly well. After a time the cast speed penalty isn't an issue and with enough strength the damage boost is certainly enough to compensate for perhaps not getting immediate access for all of the +spell and +fire damage passives you might want. As has been mentioned the mana-cheapness of the support is a godsend for Flame Totem. The main thing for me preventing it from being really strong for me is a lack of gear and other valuable supports (such as faster casting which I should really get around to grabbing) - as it is the only things that really cause me trouble are, logically enough, fire-resist things (even with Elemental Weakness kicking around)