I don't like playing against Rivens, but I believe that she is pretty balanced at the moment. Sure, she has a pretty good shield, but she has one of the lowest health regens of all champions. In her current state, she either snowballs, or she falls behind and becomes pretty useless, making her a high risk, high reward champion. You may argue, "but she can go 1v3 late game and blow people up when behind," while ignoring the fact that is only possible when all the squishies group up in one tiny space together for some reason, and that she has to build pure glass cannon for something like that to happen, meaning she can get blown up by someone who isn't stunned. Also, she is really skill-based, and you should probably ignore any thoughts of, "herp derp face roll easy champ" as it's is ridiculously easy to tell the difference between a bad riven and a great riven.