If you're considering signing up to be an "affilitate" - read your current agent contract first is you have one. My agent contract forbids signing a simultaneous contract with other brokers, including these guys.
Over the past few years I cannot tell you the number of times someone has come to one of my open houses and when I asked how they heard about it, the answer is "Redfin". Many state while they are not using a Redfin agent, they like their website because they feel it is user friendly. Personally, I didn't see anything too compelling about it.
You can assume if your listing appear on Realtor.com their is NO REASON they would not appear on these websites also.
It is very much worth your while to go to these websites as a consumer and download or read their consumer contract. This, you will find, to be very important to you. You NEED to know what the consumer is being tempted with.