So apparently they're hotfixing Demonicly Teleporting to the Dalaran Sewer pipes.
MMO-Champion BlueTracker - RIng of valor Exploit or hack?
There are a few other arena bugs they're fixing at the same time, listed in the thread, but that's the one related to this topic.

While not a breakthrough spell, it does offer more commodity and utility than one might think; I see myself putting down the circle every time I can, just in case something goes awry. Let's see:

Violet Hold: Great for recovery time for those pesky portals that open up there, as well as recovering from cyanigosa's point-blank teleport.

Loken: Makes avoiding its Nova much, much simpler... not that we do that anymore, though.

The Nexus entrance: Great for hit-and-run strikes! Just place the circle on top of the cliff, go to a lower ledge, and start burning down anyone you see. By the time they try to get to you, you can just port up.

Naxxramas entrance: Gives you a chance to avoid getting killed by those fire mages or shaman that like to knock you off the platform. You need sharp reflexes, though.

either side? is there something I'm missing in my placement of the DC, or is it actually meant to be a one way trip to the opposite side with no hope of returning (til the gates open, of course)

It's a one way trip, i guess he meant you cant go to the opposite side from either way, but it's a one way trip. I normally place it right before the pull or it will run out before the boss phase. Even if nobody dies, you can go after the boss when he teleports away to boost the DPS. it's a minor imba trick :P
You can use it to prevent a wipe when DPS is in trouble in the other side, and that alone is worth the cast.

IF you place your portal directly under where the gate closes it allows you to port to it and run to either side.


Why this happens:
The way collision detection works is the same way most geometric shapes work in 3d modeling. That is to say that every 2d polygon has 1 side and 1 side only. If you view this polygon from the front it appears with it's texture, if you view it from behind, you see nothing. The gates, and most other objects in wow work in this way. If you were to end up in the middle of geometry, in this case via demonic circle, than you can pass from the inside to the outside.

I can confirm, that you can basically switch sides every cooldown by placing the your circle directly in the gate. You can easily test this at the entrance of the violet hold. Open the gate and place your circle there. Let it close, teleport in and walk to either side.
But I'd like to warn you at Gothik - there are two holes at the angles of the gate where you cannot get out if you accidently walk in there. (Unless you teleport again after the cooldown)

Something additional:

In Warsong you can easily cap a heavily defended flag by placing your circle on the roof, jumping down, grabbing the flag and immediately teleport back. (This is even easier if you got slow fall. With this way you won't even touch the floor)

another trick is to use it as a feint
say you run up the tunnel, get to the speed buff area and place the portal
you run up snag the flag and run the the other way. people will follow you naturally, then suddenly *poof* gone and they cant see where you went

On Gluth a group I was with tried this

1 Lock stopped at end of poison tunnel and dropped his circle. Then jumped down and went to work gathering adds after he got quite a few he teleported back up to tunnel. and the adds followed him and stood at base of tunnel as he used rain of fire to keep their attention.

another lock placed his circle in the middle and gathered adds using his instant cast dots while running away from the tunnel. when he got to the other side he ported back to the middle using his circle and ran the group under the first locks rain of fire. ( soul shatter is a viable backup to help dump aggo but only once due to long CD)

Personally I use it in sarth to not only make my life avoiding walls easier but to mark the spot on the ground where its safe to stand

On Deconstructor use your demonic circle in the center and when you run away with the bomb, you can port back for increased dps.

You can no longer use it during the vortex on Malygos.

UGGG WHY?!?!?

Yeah, that's what I do.

We use a 2-camp approach for the 10-man version. On the first debuff I get I run to the side (still in range of the boss) and drop my portal. It doesn't save time on the first debuff, but it does on later ones, and it's more important to get out of the group quickly than get back into the group quickly. It's also closer to add spawn sites on my side.

Confirmed. :'(

Some thoughts:

Council - Depends on the order, but place it on where you tank one or two of the mobs, as that's where power circles will form. Run out for overload and port back instantly.

Kologarn - Kite beams away and port back to your location.

Hodir - Place circle in a general centralized location to better cross the room to fires and other buffs.

Mimiron - Circle strafe the spin-up, then port back to your location.

Vezax - Same as hodir, really, pick a general centralized location to access green/black clouds

Auraiya - Port back to cluster after fear.

Yogg - Port either to or from Sanity beams.

Razorscale & Ignis - use it to port back on the ground after a wing buffet or flame jet.

no longer here

Kologarn - useful for running away when you're being targeted by eye beams, or you can put it in the middle of the raid to port back in after you've run out.

Freya - could use it on the phases where there's Nature's Fury and Sunbeam to get back into the general vicinity of the mushrooms.

Trash before ignis - drop it in front of you, teleport when the damn tornado comes barreling through.

Though it was amusing the first time, I learned this is also a good way to take out a raid member who isn't prepared for the tornado to suddenly change direction. I throw mine up against a wall or otherwise out of everyone's way now.

I used it to avoid some repair costs while learning the iron council fight.
Just teleport through the closed door when a whipe is imminent while having aggro from any of the bosses und reset the fight
Its a sort of bug abuse but cheaper then repairing your stuff

On Thorim as part of the gauntlet group. After killing the initial mobs before heading to the gate, drop your portal under the balcony where Thorim is while facing the center of the room. While running down the hall towards Thorim you port directly down to the floor to shave off a second or two and avoid the lightning wall if it happened to go up on you.

I don't know if someone has posted this, but in VoA (archavon ) You can put in anywhere in the boss chamber. When you got hit by the "Cave in" you can port away. Also works to get out from the cloud..