Only once did I play a premade until he passed on - Johnny Zest. He had married but not had any kids when he passed (he married late in life). He died from old age. Most of the other premades I have played just to check out the clubs feature in Windenberg, or to test the fame feature before using it on my usual Sims. I also played the premade twins in university to see what was new. My favorite premades to play are the vampires. Speaking of which, I turned the garlic house in Forgotten Hollow into a haunted house. Most of the time I prefer to mess with the premade lots or premade NPC clothing rather than play them in live mode. My favorite things to do with the premade lots that already have residents is to add basements, especially hidden in garden sheds, indoor closets, hedge mazes, premade graveyard rooms, etc. I suppose I could just put it in the middle of the backyard, but if I was going to do that I would like a Beetlejuice like sign (like TS1 did).
I tend to create new saves when I have had to remove a substantial amount of mods and/or custom content due to patch updates, or if I want to build on more lots than I have left. Then there is the save I downloaded with ALL lots and NPC's removed or deleted that I sometimes clone and rebuild with gallery lots, etc. Unfortunately, the apartments don't work too well when they are bulldozed, and are much harder to rebuild than the standard lots.