I can only provide third-party anecdotes, in that I know of sellers who do use them, and find them worthwhile.
I think the best thing to do is look through your item's categories and / or others, to get an idea of how promoted listings are placed, and whether there are already promoted listings in your categories (if there are, you generally have to pay more than the other sellers to be promoted ahead of them).
I kinda dislike the whole idea of paying eBay extra money to get my listings in front of buyers, even though technically these days I don't actually pay any instertion fees, it's really only because all the other fees have been jacked up. Using a site designed to do X function, and then paying more to make doubly sure X function is provided just rubs me up the wrong way. It's how things are going, though. Basic access and tools provided for free, stuff that actually lets you do what the site is designed for with simplicity and ease - paid features. 
ETA: I wonder what will happen to promoted listings once the site has switched to product-based (collated) listings? They'd either have to become obsolete, or you'd wind up in a situation where it's a matter of who can bribe eBay with the most money to win the "buy box". 
