I'm a new shop and I did set it to use the promoted listings at first, but the sales I've had (just a few so far… I'm really just getting going!) were not through the promoted listings anyway, so I turned it off.
What seems more worthwhile to me is just getting more products up there, because then there are simply more opportunities to be seen through the free searches. That's my plan for now, at least! I hope we're all successful! :)