I did a test for a few dollars (7$ & 7$ for 2 ads), i promoted a game video from my YT channel:
1-st ad: on one broad gaming directory: was a waste of money as cpc in my case was 0.21$ which i find is way too high for promoting video.
2-nd ad: for more narrow directory: cpc was good (0.02$) but i got only a few impressions and few clicks during several days, and finally, after few days, Reddit refunded me those few dollars that were deposited probably because they decided they could not deliver ads for it in timely manner.
p.s. i have not requested to refund - they decided to do that, instead of letting slowly deposited money to go in to the ad shows.
So my opinion is not worth wasting time / money on it, at least for now, but thats just my opinion + who knows, they may improve something in the future
I thought to mention a couple of pluses though:
1. You can try it for only few dollars, no need to deposit big money to test, so many people can afford to try it.
2. They have detailed stats of ad run so you can see if ad performs good for your needs.