There isn't a simple answer for this.
Really the best way for you to determine this is to run some experiments yourself. Put promotions on some items and see if their sell rate goes up. Results from category to category and even within categories will be different so that is why experimentation is required.
In my experience, promoted listings definitely work in the right scenario.
The problem for us is for the promotion to work, one has to be spending more promoting than others promoting the "same" things, and of course we don't know what, if anything others are spending promoting. This will also change over time. For ease of math, lets pretend everyone decides that 5% is a good promoted amount, if everyone does that, then the ONLY one doing well is eBay because they're getting 5% and all the 5% promoters are just as evenly promoted as they were when they weren't promoting. That isn't likely to happen though, but over time the "average" amount people are willing to spend to get noticed might increase which will slowly drive up the % necessary to be noticed. (This concept really isn't much different than the "olden days" when one had to decide how "big" an ad to put in the paper, with the full page ad being very prohibitive $$$ wise but theroretically also very effective sales wise)
Generally I've enjoyed good results, in the order of 50% of sales from promoted items. Note that ALL of my items are promoted, with my secret formula how much I promote each group. Generally my material is grouped by age and I tend to promote the older stuff more heavily than the newer stuff. You may have noticed my black friday experiment which was effective when I generally exceeded the recommended % which I've always traditionally suggested to ignore.
Promotions, along with anything else we spend "advertising" on eBay do eat into one's margins, personally I don't think one can sell normal stuff at normal volumes with a cost of acquisition of the item above 33%, really it probably needs to be even lower with the current overall cost of selling situation, but some folks appear to be able to do it.
