I've tried them on and off for a long time. According to my stats, I've gotten 3 orders, 21 favorites, 228 views, and a whopping 51,998 impressions. All for the cost of $57.88. Not sure if it's worth it. I'm doing a trial period of them again for a week to see what happens.
But in general, I think they're a waste of money. Unless your items are extremely specific to what someone is searching for, I probably wouldn't suggest them. But you can always do a trial period. Just set the limit to $5 and see what happens.