I thought about it a bit and realized that, in a sense, I have played a few off-the-rack Sims. Occasionally I've had Sims who met and married townies, at which point, the townie came under my control. I don't think I've ever allowed them to move in without a personality and/or physical tweaking, though.
The only time I played a true, off-the-rack Sim was during the Random Challenge earlier this year, where it was forbidden to tweak anything. For me, the prospect of buying off-the-rack was truly frightening! Fortunately, in that Challenge, I got lucky. Both the Sims were easy to work, having delightful personalities and only one or two difficult traits. I'm sure it could have been much worse.