I use them from time to time. I usually set the spend to $15 for one single day. Never made money back, but have sold from one to four copies of one of my books (IIRC) with this avenue, depending on the pricing of the book. Sold one at full price on one trial, sold four on sale on another trial. I'm considering running an ad on Bookbub during an upcoming sale.
The important thing is to get good comparative authors so they can place your ad well. Liz has talked about the things she has tried on Bookbub ads in her self publishing thread, which I forget the name of. I remember her remarking that self-pub authors were better comps, and I've found the same.