There are a lot of sites that use it. Our main business site http://www.merchantequip.com uses it. Is there something specific you would like to know about it. It is completely seamless, so in most cases there's no way to know someone is using it unless they post the authnet verified seal.
Authorize.net is one of the largest payment processing sites around. Countless websites use it; both large and small. They were recommended to me by my bank when I opened a business account with them. I use them on my main e-commerce site for computer accessories, AccessoryTown.com and they are flawless. I've used them for several years and overall have been quite satisfied. I've looked into other payment processors for other businesses but so far haven't found any that would be as good but for less.