I will start with the bad things that I see in this ability. The first thing is the big cast time(3 sec). Yes some other classes may say-"This is not much!" ,but every hunter knows that one of the most important thing for us is the mobility. In PVE combat where the tank is holding the aggro you may have time to cast Stampede,but in PVP those 3 sec can make you very tasty target. The second bad thing is that when you summon the pets ,they need some time to get to the target. If the target is ranged ,they may need up to 2 or 3 seconds. With total duration of 10 sec , 2-3 sec can reduce the total Damage of the spell by 30%.The last bad thing is the huge cooldown-10min.
On the other side I see two very useful things in this spell. The first thing is the intimidation! As you can see on the video below ,when the pets are out the look like a wave of beasts and death. In PVP combat any enemy (specially in the eyes of the new players). This can buy you some time to deal with you enemy. The other thing is that if you manage to cast the spell and IF your foe is a melee one. He will take the full damage of the pets.
I still can't say is this spell useful and usable or not . Every player has his own fighting style and I think that some people will love this spell ,other will find it useless...
The truth is that the spell is very fresh and it deserves a chance.
Here is a video of the spell: