I like using Cryo Ammo a whole lot, and I have a lot of experience with it, as I use it whenever it's avaiable.
Cryo Ammo procs when your bullets hit enemy health and trigger the effect, much like all the other crowd control ammo types. The CC provided isn't as great as the CC provided by Inferno Ammo(well, what can top the AOE firedance that you get for free?), but it's CC nonetheless.
The beauty of the ammo power is that it doesn't require a cooldown, it's just active - allowing you to CC enemies while using other powers.
Squad Cryo Ammo also applies to the weapons of your squaddies, so that they freeze the bad guys they're firing at. An enemy frozen is an enemy that isn't pointing its gun in your direction.
The fact that it only works when the enemies lose their primary defenses sounds bad in theory, but in practice enemy defenses go down in a blink. If you use a weapon with a shield damage bonus, like the Assault rifle or the Submachinegun you'll see Cryo Ammo proc almost immediatly.
Likewise, slapping cryo on your pistol will make it so that you freeze Krogan, Vorcha and Varren as soon as you break through their armor. Cryo Ammo also stops their annoying health regeneration, which is a plus.
It's especially potent against "Elite" enemies with a large sum of hit-points due to the +100% damage bonus that is applied to frozen targets. This is the biggest damage bonus you can get from an ammo power, but it's only really noticeable versus elites, as normal troopers should die easily enough as it is.
Another thing worth noting is that Krogans and Varren don't do the "firedance" (The crowd control you get from Inferno/Incendiary ammo), so cryo is the only ammo power that will CC these enemies.
Generally, cryo works best for the Infiltrator as a main-ammo type, since your SMG's (barring the locust), Heavy pistols and the Viper sniper rifle work very well in Tandem with the ammo power. You can add shotguns later on to get the best ammo-proccing weapon in the game.
As a soldier or vanguard, you're probably best off using Inferno ammo/Disruptor ammo against enemies, but cryo ammo can be used to great effect as your main ammo power, as
Ryoldschool showcases here. TL;DR
- CC with no cooldown
- Squad Evolution is important
- Best against elites
- The best ammo power to use against Krogan/Varren
- Against all other enemies, Inferno reigns supreme.