When most people say "BookBub" what they mean is a BookBub feature deal, but there are also ads, and you can recommend and post reviews on the website, I think. Definitely not a scam.
Feature deals are for free or cheap books. Your book will appear in a daily e-mail sent out to LOTS (the numbers are big) readers. When I see Big Five publishers use it, it's more often for cheap books, not free.
In my subgenre (contemporary romance), a feature deal for a free book is EXPENSIVE (still cheaper than a feature deal for a non-free book, though). Many hundreds of dollars. From the sounds of it, most people do recoup their money, but not everyone does. Many recommend trying to get a BookBub--and I say try, because I think t heir acceptance rate is something like 20%--on the first book in a series, then make your money back on sell through to later books. Readers who would not otherwise hear of you may see your book in a BookBub e-mail and click. This will also cause your Amazon rankings to sore, and then other people who look at the top 100 list on Amazon and such will see your book.
(the only time I applied for a feature deal, I was rejected, but I recently used a bunch of similar services with smaller audiences than Bookbub, but similar idea, for a different book. That was Book 2 in the series, and I probably had enough sales from Book 1 to make my money back already)