In my experience, it depends on the person you get.
It's too bad you can't choose or not pay them if you're not satisfied with their work.
Anyways, I've had both - really crappy babysitters (my Sims even had to leave work earlier because that mad babysitter made my baby starve) and also really good ones: one managed to become friends with the toddler and stayed a family friend ever after.
But what I noticed with all babysitters is, they work better when the baby is in the same room as they are (so you might want to place the crib in the room where you keep your television
) and they usually only do something when the respective need is really down. For example, I hoped the babysitter would potty train my kid, but nope, he just waited until the hygiene bar was went rapidly down and then changed his diaper.