I am absolutely enjoying this expansion pack - and considering I only purchased the game a few weeks ago, this was a nice addition. I only play with Ambitions and Pets, so there may be more that I'm missing.
Anyway, my new sim has a cat named Duchess... who has been quite incredible. She began with the Genius, Hunter, and Proud traits. Through training, she has added *three* additional traits: Neat, Non-Destructive, and Adventurous. I was a little surprised that pets could have six traits, though. Additionally, she just reached level 8 hunter, and when I asked her to go hunt a beetle, it went to map mode to show me where beetles were located in the town to collect them. As a tip, giving the beetles my cat has stalked and taking them to the Science Lab to 'Donate to Science' is a nice way to make cash. And as an Inventor, my sim needs all the help he can get.
Additionally, some of the new furniture and objects appeal to my design aesthetic, so it's been nice to see it (particularly one new Abstract pattern, which I have managed to put in multiple places.) The starting house was spacious, and easily adaptable to additions without taking away from the flow of the house overmuch.
Finally, I haven't seen the Unicorn, but I have been excited by the raccoon, which Duchess fought, the rare water beetle, which Duchess brought back, and the stray dog, at which Duchess hissed.
I had been postponing getting the Sims for a long time, because I wasn't sure if I'd really like it or that I would quickly lose interest. After playing a friend's copy for some time, I can safely say that's not going to happen, and that the Pets EP was an excellent place for me to jump on.
(as an aside - love the site - been lurking for awhile - thank you for all your hard work)
