I have a kislev allied contingent. (VERY ASTONISHING MODELS)
It consists of:
Tsarina Katharina (The Ice Queen)
Tsar Boris on bear
2x boyar
36x kossar (FC)
15x Ungol Horse Archer (FC)
12x Winged Lancer (FC)
19x Gryphon Legion (FC)
I play it for 6 years now. (Either with an empire (Middenheim theme) or Brets army)
It has some HUGE advantages...... and some disadvantages.....
1st of all, if you use the tsarina in your contingent she uses MASSIVE points (500),
BUT! - You can use level 4 ice magic! - very surprising for your opponent.
- ALL CONTINGENT UNITS are STUBBORN
- You have TWO generals! (One from the main army and one from the contingent.....)
- The CONTINGENT can use the 12'' rule from BOTH armies (The main force only from the main general)
- 0-1 Restriction of the Gryphon Legion is lifted (Which means you can use them ALSO as DoW - if you don't use Bretonnia)
- Ungol Horse archers (Fast cav) HAVE BS4!!!! (cheapest fast cav in the warhammer world)
- Kossars have STAND and SHOOT (Which means they can defend TWICE when being charged!!!)
- Winged lancers and gryphon legion have glorious charge rule.... (When charging in front, it IS like from the flank... - You'll get the FLANK bonus!!)
(Now imagine you use the GL in the front and knights from the flank.....- you have a DOUBLE flank bonus

)
Disadvantages. (According to the GW Rules - NOT unofficial ones....)
- It consumes MASSIVE points (mostly to Tsar or Tsarina and i prefer the tsarina)
- Kislev DOESN'T have rare units
- Kislev DOESN'T have artillery
- Kislev CAN'T be used as main force - ONLY as a contingent.
- It can't use DoW
I use the contingent ONLY in a 3000+ battle... but THEN it can make a difference...

What they do to kislev in the 8th edtion; we'll have to wait and see....