I see Lincoln Cole had the book of the month for April. You might want to PM him and ask directly how the service works. He is a very helpful person and will almost certainly be willing to share anything he knows.
TL:DR - If it wasn't for how hard the team running the site work to promote books, I would say 100% steer clear. However, I will still offer a warning that they are crazy expensive and your results will NOT come close to paying back the cost.
My book (Raven's Peak) was book of the month over there in April, and they have about 240,000 subscribers and have been growing at a rapid clip in the last few months.
Essentially, I sort of stumbled into the program and managed to get a few different services lined up all at once. I had extra promotional budget and wanted to try something new out, and I had gotten a few reviews from them before their prices went up.
The thing is, they are expensive. Crazy expensive, and I would say 99.99% of the time you will not make your money back, nor get even a fraction of your money back. I got amazing results out of my promotion with them, and it still didn't really come that close to paying for the promotion (and that was with several extra listings as book of the day than they actually offer as part of the promotion across multiple other titles).
They have some really interesting systems in place, including daily giveaways for readers and other promotions, but in general many readers seem more interested in capitalizing on these promotions than on actually finding/supporting authors.
What I would agree with is that their site team, including Scott, put a lot of work into their system and really work hard to get results for the authors, but it would be nearly impossible to get the results you pay for considering the huge price tag on every service they offer. I would also say that most of the authors who work with them don't get as much personalized experience as I got (because of the huge price tag) and then you would have a much worse overall experience because of lack of results and lack of personalization.
That being said, they will do exactly what they promise by sending our promotions, and they certainly sell the value of their results at a very high level.
Side note* one thing I do like about them is that before you can pay for promotion, you need to get a good review. This forces you to pay for a review before any other promotion, but it also means that garbage books with a lot of problems very rarely end up getting sent out to readers.