So they are finally totally official, and can be seen here
The visual impressions have already been discussed, but what about the statline? They have the same pointcost as Mournfang Cavalry, and the differences are (both rider and mount unless otherwise noted):
editing to correct some of the errors people have pointed out
BS: who cares
S: +1/0 (ensorcelled weapons - also mean they have magic attacks)
T: +1 (due to new errata where you attack highest T of rider and mount)
Save: 1+ vs 2+ (with ironfists & heavy armour for a 16.67% points increase)
both causes fear
mournfang have D3 impact hits
skullcrushers reroll panic tests (only relevant if they break in combat)
skullcrushers are frenzied
skullscrushers have MR(1)
To sum up:
Skullcrushers have, even though they have 1 less wound, good surviveabiblity with their T5 and 1+ AS, not to forget MR(1). With a WS5 some models will even hit them on a 5+ (but rarely something you would want to charge into anyways), and most models hitting on 4+ compared to a good deal of elites hitting mournfang on 3+.
With a I5 the skullcrushers (not their mount though) will attack first in most cases (which works both offensively and defensively), or atleast simultaneously.
Skullcrushers hits most opponents on a 3+ and have S5 as opposed to the mournfangs S4 and can wound ethereals. The mourfang have D3 impact hit that does not have to roll to hit, and combined they have 1 more attack. Against ASF (pesky High Elves) this is a major advantage for the mournfang, and somewhat remedies their low I.
The frenzy of the Skullcrushers means one have to be very careful not to get re-directed. I do not believe fernzy applies to the juggernaut.
For the sake of argument we can pit 6 skullcrushers against 5 mournfang with the ironfist+heavy armour upgrade, 360 points vs 350 points:
If the mournfang charges they do 5D3S4 impact hits ~7.5S5 vs T5 ~3.75W (before saves) ~1.25W (after saves)
Skullcrushers go first which is 6x3 WS5 S5 vs T4 and a 2+ AS ~ 12 hits, 8 wounds, 4 unsaves wounds reducing mournfangs to 4 models and 1 wound. The rest of the attacks are simultaneous:
juggers: 6x2 WS5 S5 ~2.67 unsaved wounds
mournfang (including mount): 4x7 WS3 S4 hits vs T5 and 1+ AS ~ 14 hits, 4.67 wounds, 0.78 unsaved wounds.
After first round of combat the mournfangs are reduced to 3 (down by 2) models and the skullcrushers are down by a single model. Without the impact hit the Mournfang will be on the loosing side of a grind.
As someone mentioned against a speedbump of swordmasters the mournfang would get out on top, but lesson here is not to charge Skullcrushers into elven elites - they are just plain nasty!