I play mostly with the premade families, I have around 40ish played families in my current save. Each family is played for a year on the calendar with my seasons set at 14 days. My save is over two years old and I still haven't played every premade household in the game.
I give them all minor makeovers like new clothing, hair and make up, I rarely alter their genetics cause they are made the way they are for a reason and I generally don't like to mess with that, I've made one or two exceptions in my current save Darrel Charm being one of them cause he was just weird looking. :D
I love playing rotationally with the premades it allows me to get to know all the families, give them jobs, new relationships, more children and experience life with them, they are all my sims and I love them. Seeing the game as a whole evolve over time as opposed to just one family is a very satisfying experience, it's what makes the game fun for me and it keeps me playing.