How often do you use the pre-built rooms?

I use them now and then; there are a couple of kitchens and bathrooms I've used more than once. I used them a fair bit when I first started playing last year while I got a handle on building and I found them quite useful. Mostly I'd put them down and then move them around till I found a whole-house configuration I liked. Then it's remove walls and reshape rooms time. I almost never just put them down without doing some kind of fiddling with them. They are very good if you just want a furnished room to plop down, and sometimes I just yank furniture out of them so I have a certain look without searching. They work quite well if you just want something in a hurry, especially for a house of sims you don't intend to play much.