I actually don't restart too often, I tend to get attached to my sims and try to stay with a family/save file for as long as possible. But, any time a new major pack gets released that adds some cool and exciting new gameplay, I normally have to start fresh and create a whole new sim/save file to go through the new content with. Sometimes I plan on trying to incorporate the new content into my current save if I happen to have a YA or close sim that I don't already have a plan for, but usually it feels so much more clean if I start over and just focus on that new pack.