With each generation of Sims (Sims 1, Sims 2, Sims 3) I've always thought that it's not really Sims until you get pets! So I have been waiting for this pack for some time!
Pets are the best addition you can have, they're fun, you can train them, you care for them, they can have their own babies!
When I first started playing, I wanted one of every pet, so I got a cat, 2 dogs, a womrat and eventually a horse, but I found that looking after them all became a bit difficult, especially looking after the horse, so I had to give a few up for adoption.
It really is a great game, though. The traits the pets have play an important part in the game, as it controls your pets behaviour, and they can even learn new traits if you use the 'Reinforcment' action with your pet and praise or scold them for things they do.
Overall, it's the best expansion pack, but I need to get used to the pets needs before I have a whole house full of them! :)