I have read here (afflict sticky) and kinda got from ej forums that haunt should be used on cd? Did I understand this correctly? is it of such a high priority as to cast as soon as it is off cd instead of when it has like 1.5 sec remaining on target?
Yes because of the long travel time. However, you don't want to cancel a cast for something else to cast Haunt. That's why the cooldown is much shorter than the duration.
Casting Haunt on CD isn't the important part, it's keeping the Haunt debuff up that is.
Haunt has one of the slowest travel times in the game, so when casting from max range it actually takes a noticable amount of time to hit the target. There's a 4 second window from when your Haunt comes off CD to when the next needs to hit the target. If you're in the middle of a cast, finish up and then Haunt, as you should have time. However, in instances where you're about to need to move (i.e. Avoiding a Shadow Crash in Cho'Gall), Haunt takes priority. If there's no Haunt on the target while you're running around, there's a problem.
depends, if ur at max range it is a good idea to cast when its off CD but if ur in melee range like on chogall maybe just cast so it hits just before it expires to maximise dps, but dont ever let it run out
Edit: Stupid post incoming... please ignore!
If you need to move, use Felflame as you move. Note that this is the only time FF should be used to refresh Haunt. Using a GCD to add half the duration isn't worth it.
The best use I've found for FF is killing low hp mobs for things like Steamwheedle Cartel rep :-)
On topic: Haunt on cooldown, but finish what you're casting first (as mentioned by other posters).