First of all, 15 heroic badges for 53 defense rating. By the time you have 15 heroic badges you should be getting close to the defense cap anyway. Granted this does provide more flexibility in gearing decisions. What concerns me, and everyone else I've read is that its a proc on Icy Touch. Even if this procs 100% of the time you use icy touch, there is always the possibility it wears off prior to your next icy touch and all of a sudden you are critable for a short duration. If you are in the PTR please post feedback on this item to be defense on equip with no proc criteria.
Here's what I'm most likely to go get as an Unholy Tank. Until I can trust the defense sigil I'd much rather have a huge buff to my bread and butter strike. 203 to scourge is looking a lot nicer than 90 to blood strike.
Our Frosty friends will like Frozen Conscience, same damage but applies to Icy Touch which they buff through talents.
The Wild Buck, I'm not sold on. I think I'd rather keep dark rider with its 90 damage to Heart Strike than this.
Looks like Blizzard is batting .500 coming out of the gate with the new sigils. I'm sure our friends over at EJ will do the math on Wild Buck to see if it's ever worth taking. The frost and unholy focused sigils look sweet though.