I've used this site a couple of times for promoting books through email, and it's one of the newer sites that I've seen around the block. This can actually be great in reaching a new audience of interested readers, but it also means that it likely has a smaller subscriber base than many competitors out there.
BookPebble is super easy to actually use, and this is huge when it comes to signing up for a promotional service. There are a lot of services out there that will charge you twenty-five bucks to submit your book to a lot of free sites, and authors use them all the time because it can get extremely cumbersome to repeatedly sign the same book up for promotions.
BookPebble's signup reminds me a lot of the eBookSoda signup form and is just as easy to use. You fill in the details, hit submit, and then get a confirmation that your promotion is all set up for the day you want it to run.
Nothing too major or out of the ordinary: Under 5 bucks, full length, great cover, and more than five reviews. Error free is a plus, and by now you should have an amazing book description and opening anyway so that shouldn't be a problem.
If anything these requirements are under industry standard, which means you're more likely to get picked if you do submit. Check out this link if you want to submit to them and find out more about their overall requirements.
It isn't too expensive, and you can usually run a promotion here for under ten dollars. That isn't too bad, considering you're going to get a fair number of clicks out of it. You can also add a facebook and twitter promotion for extra money, but in general I don't think those will do as much good. Check out their genre listings and prices here.
They are growing, which is why they are free, but that brings up the problem of submitting to a new list that doesn't have a lot of readers. Since it's new, it might actually get better turnaround of clicks from its users (because new users are traditionally more invested in a list and suffer less atrophy) but it's still a small number of people who are getting sent to.
Still, it's cheap, and I would definitely recommend checking them out and submitting a book. They have some requirements you'll have to meet, but many people will be able to get over these and get their book out there.
I have run a few promotions with them in the past and I still consider them to be a staple promotion site when I'm trying out a new deal or discount, but I wouldn't base a promotion around them. They are a great supplement, but if you don't stack them with other promotions then don't expect much.
My Rating: This one manages to earn a 3-star rating, which is definitely good considering. They used to be free, but I can understand and appreciate that they have to charge now. They aren't too expensive, and if you happen to have a good book that is a match for their audience then they can get you quite a bit. Still, don't put your expectations too high.