So having read up on the psychic powers yet again did my eyes fall on the lovely biomancy power known as constrict... But that was at first glance.

Now that I have read the errata yet another time (that they now can make a WP check to act as nothing are happening), and that it are going to be a power that you have to keep running for a long long while to do any harm (twice T bonus before there is a real effect), are the power then useful or just useless?

It appears to not be a sustained power, meaning that the psyker can choke the guy and then next round choke his friend, and start to lock down the entire group if he wants.

Yes, the best you can hope for is for 30-something Toughness and so they'd take 10 rounds before they pass out (by which time they've likely broken free) but the combat usage isn't this. It's zapping everyone's throats shut so they can't shoot at you.

Very worth it. Though in combat, as Pixels said, you shouldn't be using it to outright try and kill people (though it can). It is a punishing and a powerfully inhibiting ability.

Think of it this way, you are forcing a WP test for them not to freak out about suffocating. If they pass this test, they act normally (but are stiff suffering suffocation). If they do not pass this free action, they spend their whole turn making a toughness test. If they pass, they clear their windpipe and wham. Next turn they act normal and are free of the pysker spell. However, if they fail, they must do the same thing again next turn; (does not need to be sustained!) and so forth.

A fantastic way of distracting or debuffing an enemy group or even a singular enemy. I would encourage its use on enemy snipers or heavy weapons teams, or even on lieutenants.

I have seen the light of the closed windpipe, especially since it is a half stealthy way to kill people on top of the things there have been said, suffocating men can´t yell after help after all, and can silence those there are yelling after help.