For me, they're a little lacking. Don't get me wrong, I love how cute some of them are and I really like the fact they made something for everyone and every Sim. However... I was expecting a little more than a glorified death flower.
The only things I can do with my familiars on my Spellcaster is dismiss them or rename them, I can only have one at a time (which is ok, but imagine having a dragon, bunnerfly and leaf bat at the same time?!) and as far as I know, they only do 3 things total: Save your sim from death (this I haven't tested as I dismissed my familiar before my Sim died of magical overload), boost exp gain rate on your spellcaster (not 100% sure about this), and "talk" via notifications that you may or may not miss in the middle of the game.
I kinda wanted to be able to pet them and feed them (which I understand wouldn't work for all of them), have them do more than just be like the streaming drone following your Sim around. As a Sims 1 player who loved the dragons in that game, I was really excited when I learned dragons would be an option. I spent like 3 days in-game waiting for the dragon to come up, and while it's a perfect dragon for Sims 4's art style, it could've been so much more than it is.
However, this is just what I've experienced, and I'm curious what your opinions are on the subject, especially as I haven't found any threads about them!
Twists In Time And Space
(Updated December 2nd 2018. New discord server!)
(Updated August 12th 2019)