As many others have pointed out it does affect their moods some when doing something they like, vs doing something they dislike. The difference for me is that they can do things for longer if they like it without getting tense, bored or tired. I have also noticed that they won't autonomously do things they dislike - so for instance a sim that dislike gaming won't just leave a conversation to go play on the computer.
Other than that it is fairly pointless as it stands today. Why we have fav. colours is for instance completely baffling to me (why would I waste any of my 20 likes/dislikes on something that means absolutely nothing?)
My hope is that the likes dislikes will be expanded to mean something in the future, such as using it for turn ons and turn offs - now THAT would be useful!