I joined the program about 9 months ago when it first started on live.com. It has got so popular that they made a commercial for it. Here is the first Bing.com Cashback commercial.
Also I would like to note that you do not get any cash back until your account has $5 or more in it. They will send you a check or preferably by paypal. Paypal is pretty much getting paid within 24 hours. The cash back percentage changes almost daily it seems by some retailers. Over the 9 months that I have been in the program I have only seen 2 stores raise their cashback percentage. Most lower it every 3 months. There are occasional specials where they have a higher percentage for a 24 hour period. If you shop online at major retailers you have nothing to lose but cash back by joining the cash back program.