Welcome to the forum,
@Kaesa!
I've only scratched the surface with this in the couple of hours that I've played, and that's a good thing. Something I read on the official forum was that if you give a child the "good" trait, for example, when aging up from child to teen, you'll get a big jump in the empathy value. I think if you look at Carl's article on Character Values, you can find out which traits provide which boosts.
Other than trait influences, it looks like gains are in very small increments, which I like. It's going to take a lot of interactions to get to any single trait, but if you start from a toddler, you'll have time to get all 5 traits, I think.
Overall, I think the pack makes teens and children more unpredictable. I've had one teen get a mood swing, and I loved the thumbs-down that she gave to her adult sister's suggestion. Two of my children have made messes, and one asked about eating dirt. I do wish the child mess was different from the toddler's -- more like toys scattered around rather than spilled paint. But the child did stop when his mother asked him to.
I would say that this pack is a must-buy for anyone planning to do the Ten-Gen Townie Dynasty or any type of game in which you control only one Sim. My parent is now able to put children in bed (very similar to tucking in a toddler, quite sweet), and has reached the parenting level where she can influence a child to do homework.
I like the furniture that comes with the pack, the couches especially and the new shower. CAS is okay, but nothing I got excited about. I haven't tried the school projects yet or the volunteer work, but I'm looking forward to those. The real value is in the interactions. There's a big difference between my making a teen mop the bathroom floor and my Sim making her do it, even if I'm the one choosing the action for my Sim. And I'm looking forward to the random phases and pop-up questions.
On the whole, I'd say that if you want more chaos in your Sim's families and you really enjoyed what was done with toddlers, then this is the pack to get.