I might be able to jump in here and add some value.
They took a risk. Plain and simple. The same exact risk everyone is now discussing.
I'm not associated with them in any shape or form but I watched them over the past 6+ months.
Long and short, they took a risk. A big one. They put a LOT of advertising dollars out there. A *LOT*. Like, a whole crapton and a half. They had google ads (beyond the typical facebook ads most of the free promotion sites do) and bing ads. And they put a lot of money into it.
How do I know? Because their ads never went away. And, one has to reasonably assume when you are bidding on high CPC (cost per click) terms such as "free kindle books", you will not only get lots of clicks, but, it's going to cost a lot of dough. In short, when you put google ads up, every time someone clicks.. that takes away from the amount of money that your campaign has available for the day. And to last all day long.. day after day... it was a big pot.
Did it pay off? I'd say so. They have a tremendous growth :http://www.quantcast.com/bookbub.com
That's all from advertising.
If I were an author... would I think it was worth it? Hell, yes. To have an audience that is that targeted and large... I think they have a tremendous product.
But, first... I would ensure that every marketing aspect of your book page on Amazon/insert-your-seller-here was in the best light it could possibly be. THEN take up costly opportunities like this.