I haven't run any campaigns myself, but out of curiosity, I ran a search to look for promoted listings - my adblocker (on both Chrome and Firefox) usually prevents any from showing up in the sidebar(s). I switched from ending soonest (my default sort order) to best match - no promoted listings in normal search results, but there were two at the very end of the search results, separated from the rest (like how international items often are).

I have seen a different style of promoted listings, where there is a few products at the top of the page in a scrolling gallery, but they're always from the same store, so not promoted listings as such, but a showcase to promote a specific (always big-name) store and I think that's something that's not accessible to the majority of sellers (i.e. it'd be part of the exclusive advertisting deals they offer big name stores when they entice them here), so even with promoted listings, you don't really get 'headlining act', so to speak. 

eBay also decides how relevant your item is to the buyer's search terms, and then if there are competing listings, they will (probably) go for the one who is paying the most. 

I think if you sell in a relatively competitive category (but perhaps not flooded, because that increases the chance there'll be a lot of other sellers competing with promoted listings as well), but have a really eye-catching product / product images, it might be worth an extra percent or two - based on the fact that I know my adblockers (for some unknown reason) don't always block the sidebar advertisements, and every now and then one of the images will jump out at me and I click to look at something I wasn't even really searching for (eg say I'm searching for crystal beads, ads in the sidebar just seem to take the 'crystal' bit and will show me things like crystal glasses). 

It's good that you only pay if someone who clicked on an ad goes on to buy the item, but in all honesty how can you be certain they found you that way, you only have eBay's stats to go on, and the integrity of their data is questionable at times.... o_o