I love it so far. I haven't gotten to do everything yet.
I am a real life Dog Person. I created two dogs. The skittish trait for dogs is kinda annoying when pets freaked out over someone for no reason. Also, you may want to get the non-destructive trait for the pets or have a toy box. I also like the "Pee On" interaction. My dog Lucy pee'd on a paparazzi because I hated them. The gross part is that she "played" in the puddle of pee.
I haven't really play around Cats that much. I am not a Cat person, and if I were a girl, then I won't be a crazy cat lady.
The thing I like most is the horses and the running/jumping skill. I have my sim Sugar Milton ride all over town looking for an unicorn (Which I sadly haven't found yet, but I found 4 wild horses in a fishing spot. I really need to go on a race with my horse. I never had a horse, but I find the horses the best feature of the Sims 3: Pets.
I need to go do some other stuff like have a unicorn, have my pets breed, and other stuff.