Compare the tier peice with what you would use otherwise to see what you like. Ill probably use legs and gloves.

[Conqueror's Nightsong Cover] vs [Collar of the Wyrmhunter]

[Conqueror's Nightsong Vestments] vs [Robes of the Umbral Brute]

[Conqueror's Nightsong Mantle] vs [Shoulderpads of Dormant Energies]

[Conqueror's Nightsong Gloves] vs [Handwraps of the Vigilant]

[Conqueror's Nightsong Trousers] vs [Leggings of the Enslaved Idol]

I think a lot of has to do with what non-set items you'll be using. When you look at the just of it, they're basically equal except one has crit and the other has hit. You gain a socket, you gain stats. But what stats do you need to make up, hit or crit? Sure the gloves of vigilant have 20 more crit than tier has hit, but hit is also worth a LOT more if you're not capped.

I haven't really made up my mind on which I will use for BiS items, but you can't really plan that out at the moment, it's really best to just get your 2pc and worry about the rest later. On our token mages and Rogues have an amazing four peice and I have not been trying to deny that to them. Not sure about DK's.

With the upcoming spirit change, my BiS list has changed to this:

Head - [Collar of the Wyrmhunter]
Shoulders - [Conqueror's Nightsong Mantle]
Chest - [Conqueror's Nightsong Vestments]
Wrists - Grasps of Reason
Hands - [Handwraps of the Vigilant]
Waist - [Cord of the White Dawn]
Legs - [Leggings of the Enslaved Idol]
Feet - Treads of the False Oracle
Ring 1 - Conductive Seal
Ring 2 - [Shimmering Seal]
Trinket 1 - [Illustration of the Dragon Soul]
Trinket 2 - [Flare of the Heavens]
MH Weapon - [Staff of Endless Winter]
Offhand - n/a
Ranged/Relic/etc. - [Idol of the Shooting Star]
Neck - [Pendant of Fiery Havoc]
Back - [Drape of Mortal Downfall]

250 hit rating set, and either tuskar's + a 16 hit gem, or Icewalker to get capped.

I would enjoy some more precise dps modeling to get new endgame stat values, but for now I am using values from the T7.5 level of gear progression on Simcraft and modifying them for a best guess now. I inflated the value of crit and spirit to accomodate changes in the game to make my estimates. Loot Rank for WotLK.

[Purified Twilight Opal] in blue sockets, [Reckless Monarch Topaz] in yellows, [Runed Scarlet Ruby] in reds. For the legs, gloves, and weapon I plan to break the socket bonuses and just use SP gems.

I don't know how often the Flare will proc, but if it is anything like the trinket off of Heroic Sapph I would lean more towards the Eye from Razorscale, the 'constant' 125 SP should easily outweigh the DoT proc over the course of most fights. Couple other things I would change around that might turn out more efficient numbers, but I would have to play around with it.

edit: i stand corrected

The proper flare of the heavens stats are 120 crit rating and a 850 sp/10 sec proc (with an assumed 45 sec ICD).

Source: The World of Warcraft Armory

Im gonna go with this gear, maybe i change something.

Head: Collar of the Wyrmhunter
Neck: Pendant of Fiery Havoc
Shoulders: Mantle of the Eternal Sentinel
Back: Pennant Cloak
Chest: Robes of the Umbral Brute
Bracers: Grasps of Reason
Gloves: Conqueror's Nightsong Gloves
Belt: Sash of Ancient Power
Legs: Conqueror's Nightsong Trousers
Boots: Treads of the False Oracle
Ring 1: Signet of Manifested Pain
Ring 2:Pyrelight Circle
Trinket 1: Eye of the Broodmother
Trinket 2: Scale of Fates / Illustration of the Dragon Soul
Mainhand: Soulscribe
Offhand: Ironmender
Idol: Idol of the Shooting Star

Why are you going for Eye of the Broodmother when it is bound to have lower spell power overall than Flare of the Heavens (Assuming there is actually a 45sec internal cooldown, with a high proc chance)

850 / 4,5 = 188,8 SP vs. Broodmother's 125.
I mean, it also has lower crit rating.

Here is a list of items, rated with start priorities.

The stat prios are taken from Gray Matter in one of his posts.

'Hit = 1.48, SP = 1, Haste = 0.84, Crit = 0.80, Int = 0.35, Spirit = 0.15.'

The link is here.

Please just to note, that you shouldnt follow the exact rating of the items on the list, since Hit loses anything that goes for it as soon as you hit the cap, this would go for the gemming too, ofcourse.

Graylo hasn't updated his lists to the new spirit value (which is now 0.3), and they completely ignore the 2T8 and 4T8 bonuses. While we still disagree on the value of 4T8 (I value it much lower than Gray) we know that it is definitely non-negligible. They're still good lists, but if you're not versed enough to correct for these two things in your head you should use them very carefully.

I already got Extract of Necromantic Power, it just seems its not procc that much to be worth have equipped, but maybe it is.

He is not only ignoring that but both dots aswell. This overvalues haste and crit. Only the hit:sp:int conversion could be accurate.