Wargs... random puppy dogs... sometimes. The thing about Wargs is that if you use them properly, then they are a serious menace, if you don't, then you have just wasted however many points you spent on them. I will start with Warg Riders.
The increased strength of the Warg means that on the charge, then they can really dish it out. They do, however, fall down against a line of swordsmen backed up by spears. (As do all cavalry) But as NoW pointed out, if you can get your Warg Riders around the back of the Good Side, then they will have to divert forces to protect their rear, and if you can time this correctly with the attack of an infantry force from the front, then watch the Good side crumble. Also, you can set you infantry up with one flank anchored to a piece of impassable terrain, and deploy your Warg Riders on the other side of the impassable terrain. if your opponent chooses to concentrate his force on just one of your two parts, then he will be hit in the flank by the other. While this may not be as devestating as in Fantasy or 40K, a flank charge can still be a deadly weapon. If your opponent chooses to split his forces, then if your Warg riders get the charge, then they should be able to beat their half rather easily, and then get round to help the infantry. It is also worth choosing a Warg Captain to lead your Wargs, in order to declare Heroic moves, so you get the charge more often. Also, in the new rules, Wargs can keep on fighting once their rider is killed, the only downside of this being that they have to take Courage tests on a courage of 2 every turn, so they will not last too long, but hopefully long enough to wipe out a couple more of those irritating Free Peoples.
White/Wild Wargs (I will refer to them as White Wargs.)
Very useful indeed, especially with a 10" move. The new rules are not very clear about wether these Wargs are cavalry, but I will treat them as cavalry for the duration of this article. They can be extremely useful, e.g. you can countercharge cavalry so they don't get the charge bonus, and if you have an entire army of White Wargs, then a cavalry army means nothing. And the Chieftains! for those of your who don't have the new rules, their profile is as follows.
F: 5
S: 6
D: 5
A: 2
W: 3
C:3
M: 1
W: 3
F: 1
Pts 75
OUCH! 6 strikes on the charge, and the count as monstrous mounts, because they have a strength of 6! So is you have, say, 20 White Wargs, and 5 Chieftains against a cavalry army, then the Warg Chieftains, whenever thay get the charge, can go around dismounting the enemy cavalry.
Go the Puppy Dogs!