i play a BH tank and the interrupt is out of gcd. I have many times hit it with a 2 or 3 sec cast 3/4 of the way through and it doesn't interrupt. Very annoying if the npc is a healer.
I have noticed the interrupt b4 the animation. Yes. But i've only seen this when i'm anticipating a spell about the be cast, so i don't use any abilities. So i wait a sec or two so i can catch it. Otherwise it doesn't get off. I only do this if i absolutely need to. Healers and such.
In other cases, the enemy will use a channeled ability. Such as a directional AE. I hit my interrupt as soon as i see it. But it doesn't actually interrupt till the channeled ability is at 50% or 75%. And if i don't get it off the bat, then it doesn't interrupt the actual ability at all.
Bottom line is, if i'm not using abilities, the interrupt goes off as soon as i hit. But, if i try to use it following an ability it takes 1 sometimes 2 seconds to actually go off due to waiting for the previous animation to finish. I have this problem as well with situational abilities (rail shot and shoulder slam) and medpacks. Been a few times when i klik a medpack and .5 - 1 sec later and i'm dead. I generally run 20 - 30 fps.
unacceptable imo