As an infiltrator, you're deadliest at long range with a sniper rifle.
I find it most effective to use the cloak (1) to safely peek out of cover to line up headshots with the extra damage bonus, and (2) to reposition if the enemies get too close.
This tactic works great in multiplayer, where an infiltrator can solo pretty much any wave without a timed objective by kiting enemies around and only taking shots from a safe distance.
gaming.stackexchange.com/users/11742/hammar
Cloaking provides a defensive benefit of being able to revive squad memebers without attracting fire.
gaming.stackexchange.com/users/21078/ProximateTen
Having played a bit further I have found cloak to be a bit more useful.
You can get a perk that allows you to use one power and still remain cloaked.
So you can cloak, get into a good position, take out a few enemies by using a power that has a blast range e.g. grenades, and then retreat.
I haven't found it to actually provide much benefit for headshots as the sniper rifle's time dilation makes this easy enough without cloaking.
gaming.stackexchange.com/users/7019/row1
Sneaking up behind is generally a bad idea, unless there's only one enemy left.
Using cloak for easier (and more damaging) headshots is the best use I've found so far, but you can refine it a little.
It's very useful for getting into a better position when under heavy fire, and it's great for reaching otherwise impossible angles and eliminating that annoying biotic in the back.
Other than that, just make sure you activate it as often as possible (unless you also maxed incinerate), even if you don't take advantage of being invisible the extra damage is reason enough to use it (though it's not really reason enough to max the skill, sorry =)
gaming.stackexchange.com/users/3172/Malabarba
I recommend using Cloak Shot.
Deploy cloak and line up with sniper rifle.
The last perk for Tactical Cloak improves sniper damage by 40%.
Also useful is Incinerate, for armored enemies, useful when you have low ammo and just blasting Husks when they get too close.
This is very important.
Get your chosen sniper rifle to lvl5 VERY quickly; I can't stress this enough.
You'll be one-shotting enemies like nothing.
If you're going to buy alternate powers, I recommend Warp Ammo or Energy Drain.
E. Drain comes in handy when you're low on health.
You can drain enemies shields/health and gain it back.
Warp Ammo does really great damage, even better than Disruptor Ammo (I don't know much about it, so try another guide).
Now onto the important part: choosing your sniper rifle.
Everyone has preference: Viper for fire rate, Widow/Mantis for power, etc.
Personally, I recommend the Black Widow.
It has the power of a Widow (okay not entirely; it's weaker than the widow
but it's still able to one-shot almost as good) and it has 3 shots instead of 1 before you reload.
Also, the M-98 Widow and the Javelin.
If you've played the Widow, you've played the Javelin; they're that similar in stats.
If you're the type of player that wants more fire-rate than power, the Viperor Inscisor would be good for you.
They do decent damage and have a good firing rate.
Tactical Cloak is your best friend as it's used to escape fire.
Cloak Shot (DAT damage boost)
sneak-up for melee damage
(I guarantee you'll use this power the most).
The drawback is that, unlike in ME2, the AI is smarter and they still can shoot you if you cloak up close to them.
gaming.stackexchange.com/users/56019/Devlin
I find it is best used to retreat while damaged so you can be safe until your shields recharge.
I have found it occasionally useful for the damage boost
but only when the game gets stingy with thermal clips.
So.
basically I think it's useful for being a coward.
gaming.stackexchange.com/users/96269/Sir Vanscoy
I chose the infiltrator class and have maxed out the cloaking ability. Overall the class and their cloak ability still seems disappointing when compared to some of the other class abilities.  So far I have been using cloak to get easy headshots with the sniper rife at medium range or to sneak up behind a small group of enemies and melee them. But both of these strategies mean that I can only take out ~1 enemy before becoming uncloaked.  What is the most effective strategy for cloaking?
How do you use cloak effectively?
gaming.stackexchange.com
<mass-effect-3>
gaming.stackexchange.com/questions/53813
gaming.stackexchange.com/users/7019/row1
Infiltrators are deadliest at long range with a snifler rifle. Cloaking provides a defensive benefit so you can get into a good position and take a few enemies out using a power with a blast range. A single ability, like the cloak, won't be really effective if won't combined with strategic gameplay and proper gear. The community proposes few ways that cloak can be properly used. 
Few strategies are provided.