In terms of number of affiliates, SAS is lagging behind CJ or Linksale. But though they lack in numbers, they can compete with the quality of affiliates. Also, with the lower setup cost and monthly fees, its definitely a good place to start a new program.
I'm an affiliate of Shareasale, used it about a year and Loving it!
Unlike CJ who loves to auto decline, SAS display merchant which have auto approve.
It speeds up the process choosing the merchant I want to promote.