Just to throw my two cents in...
Sinosleep has demonstrated that enemies have more health than defense, and that the damage bonuses from ammo are effective against that defense type only. Thus, incendiary ammo would have a 30% bonus against shields only, not health. Thus, for most single-defense enemies, you'll tear through their defenses far faster than you'll go through their health.
Thus, I've taken Simbacca's route and choose ammo based on their CC effects. That being said, I do enjoy squad disruptor ammo, but I'm giving cryo ammo a try on my infiltrator, and I have to say that when you're going up against enemies that immediate rush to melee - krogan, FENRIS, husks, Loki, klixen, varren - it's very handy to have because it allows you to continue holding the line. There're few things more satisfying than watching that charging varren frozen just as it reaches your feet.
Using a OSOK Infiltrator is very different than using a soldier or vanguard: she doesn't have the speed, brute force, or ammo reserves of a solider, nor does she have the mobility or close-contact power of the vanguard. Quite frankly, those two classes don't need squad cryo as much as the infiltrator does: there are two very important factors in combat that a lot of people tend not to consider: time and distance. To some extent, the soldier and vanguard can play around with these, but the infiltrator can't slow down time or cross distances in a blink. Thus, she needs many ways to get from point A to point B, and while she does have those many ways, squad cryo ammo is one of my favorites: why stop time when you can get to a crucial spot while your enemies are frozen solid (then cloak when they're not)?
Against intelligent enemies who duck for cover, it's also very nice because it takes them out of combat about as long as an overloaded weapon would. This isn't to say that cryo ammo will help you kill things faster, but it does allow you quite a bit of breathing room. Need to get to a certain position? Freeze the target, move in, shatter, take over the position. Sure, you could kill the target before moving in, but if you do that, one enemy from that pack you've so nicely pushed into that cramped area you call 'the barrel' might get it into his head to move in there before you do.
Like most of the pro-cryo posters on this thread, I give cryo ammo to my squad mates and reserve the defense-tearing ammo for myself. What I tend to do is strip defenses away from targets as I see them, then move on to the next one as my squad mates freeze the crap out of them.