We didn't use one at all for DS1, but it was a life-saver with DS2. He was a very unsettled baby and the only places he would settle were the sling (but only while I was moving around) or the swing. Needless to say I couldn't have survived without it. It really just depends on the baby I think.

Oh and BTW, DS2 is a fantastic sleeper now. He goes down without any fuss for a day sleep and at night, so it certainly didn't do any harm there.