I've been to Rai Lay three times now and I can't get myself to order anything else but the Kao Soy because it's that good! In my humble opinion it's their Franchise entree and the bartenders concurred because people love to order it. I ordered mine with both chicken and tofu because, well, since the entree has the word soy in it, you better include some bean curd.
Please see Rich's earlier scouting report for a detailed explanation of Kao Soy and its origins in northern Thailand. Overall, if you are tired of Pad Thai and want to order an authentic Thai noodle dish, you can't go wrong with Rai Lay's Kao Soy.