Since X & Y were just released a little under 2 months ago, it is hard to find the percents of wild Pokemon appearing. Most sites use the indicators "Common", "Uncommon", "Rare" and "Very Rare".
The PokéRadar works largely the same as before. When you activate it, several areas of grass will shake. When you go to them, a Pokémon will appear. Defeat it and it will commence and continue a chain and more areas will shake. The more a patch of grass shakes, the rarer the Pokémon.
So yes, the more violently the grass shakes, the rarer the Pokemon.