Robin Reads is fairly difficult to get accepted into, but they are another site that actually has fairly good results. Though, they have begun to slip so it's hard to tell what will happen next with them.
They have a fairly large audience, but it seems to be declining rather than growing. This is a trend in some of the older sites that can actually hurt your chances of generating sales.
They are expensive, which is doubly bad considering their results have been slipping. I have read a few comparisons that say this is one to steer clear of, but they have always been very professional and fairly successful for me.
This company is hard to get accepted into and has firm policies on how often an author can submit and what kinds of books they will accept.
They used to have really good results and were near the top of the book marketing ladder, but they've started to slip recently and don't seem to be doing as good of a job of maintaining their position.