Is it me, or is this seriously the best crowd control ever.

Almost instantly cast, some rays even feature longer range. Stacks well with mastery and destruction of the other sigils.

You just try to maximize the freezing time and then you can have fun with any other Sigil that doesn't do damage over time.

Added plus: it rips right through the shields of those pesky enemy spell casters.

Heck Yes!

In my opinion, Ice sigil is fantastic!!! Ice does the most base damage as a destruction spell, can freeze enemies (even bosses) solid as a control spell, and applies a freezing aura that slows down enemies as a mastery spell. Also, the control spell is not broken when applying a mastery spell!

Ice control rays are amazing, especially if you can find one that extends your range. Also, ray shapes can be used to stop arrows in flight.

Caution, later on in the game, you will start to encounter insanely powerful demons (I'm looking at you Lost Zasad Map) that can one shot you if you are in melee/ short range distance so be careful.

I might have been unlucky so far, but with the ray shapes at my disposal I can't freeze enemies long enough for me to do anything substantial, so I'm relying more on AoE for crowd control (at least until I can craft a better ray spell). Pools and areas are pretty cool - literally :D

As for Ice spells as damage dealers, I always thought Ice is best as a control-based sigil and never bothered too much with destruction. I'll have to experiment with targeted destruction spells to complement my AoE control ones.
I might use an AoE destruction spell later, when I get to craft a good control ray, but AoE control spells are too good to switch out.

Any other advice for effective Ice usage?

I had that problem for the first World or so until I somewhat grasped the crafting mechanic.
The trick is to get a good mix of augments.

The basics:

High Control means longer freezing period.
High Destruction means higher damage when shattered.
(High Mastery means higher Critical Damage in combination with any Debuff Spell but let's not get into detail here)

Now there is a truly marvellous property called
+ Status Effect Duration which additionally improves the length of the freeze.


So what you are look for is a highly destructive Control shape with a Status Effect Duration Bonus.
If you have the right high end ray shape you can have 5 of those, by the way.

You can try and look for Control augments with a "Paired" Prefix (Destruction and Control) and try to upgrade those to get a Duration bonus.

The intermediate:

If you are lazy like me you can summon undead minions that freeze enemies when hitting them. All you have to do is have a Necromancer (Sigil &) Critical Control effect on an enemy and kill it with Ice damage.
The summoned minion will freeze enemies in their spot, while you don't need to bother much any more. The advanced:
To achieve supra-damage (aka Overkills) frozen Status doesn't work peculiarly well with masteries that applies damage over time.
So either you apply a mastery that doesn't do that yourself or have a backup plan (like Delirum Control or Kinetic Mastery) when some enemies are hit by a damage over time kind of spell.

My favourite combination I tried out is
Control Ice Targeted Ray
Destruction Ice AOE Pool (To have some sort of direct Ice damage in case I want that minion type)
Unused Nova (but probably control attuned if you get into a difficult spot enemies freeze around you, could be handy, never needed it)

Destruction Kinetic Targeted Missile (Always charged for criticial damage with Control - Ice Shatterer and Mastery Overkill Bonus)
Mastery Kinetic AOE Pool (Alternative to freezing things, except casters and long range idiots still are a nuisance)
Unused Nova (probably Mastery for the same reason as the aforementioned)

Mastery Necromancy Targeted Missile
Control Necromancy (For the rare boss that needs some Frozen restrain orders)
Destruction Nova (For the rare boss that needs cannon or rather skeleton fodder to keep him/her occupied.)

Agile Shield to jump around like a fool just for luls and because I never liked galavanize anyway...

What's the catch?
No Synergy Specials are possible

What's the gist?
Mastery undead minions immobilze enemies, Ice Freezes and provide extra damage, Kinetic destruction pushes enemies away and supposedly does the most critical bonus damage = EXTREMLY HIGH OVERKILLS (about 2000 - 3000) in the first level. On a rare occasion I had an 10k Damage with Rodgiro still on the run.
With this combination you are versatile, have excellent crowd control, enemies are stuck, frozen or both.Undead apply mastery all you have to do is switch from Freeze Ray to Kinectic Missile and kill everything frozen solid and too close to you to be a danger.

Basics
I know about the status effect duration, but I had no idea how to get a good one. Thanks for the advice.

Intermediate/Advanced
I am missing the Necromancy sigil - I'm still in the early-to-mid game, and I guess I won't be able to use Synergy Specials until later. Oh, well. It will be the next sigil I unlock :D
As for Kinesis, targeted destruction spells are something I have to try as well - it looks like I've been deceived by their pretty low base damage. Kinetic mastery-attuned area and pool spells are insane and I will never ever use anything else in the AoE slot.

Great in-depth post, you deserve a +1!