The 212 Plus is a very good cooler for the price. However, it can only handle mild to moderate overclocks, as it is one of the smaller heat sinks. A bigger badder "big air" cooler will be required to get better temps at higher overclocks. Cooler Master V6 GT, Thermaltake Frio, Zalman 9900 MAX, Noctua NH-D14, and Thermalright Silver Arrow will all out-perform the 212 Plus, but they are also more expensive. The Hydro series from Corsair are roughly equal to the top level of "big air" coolers in terms of cooling performance.
Any cooler that relies on air can only get as cool as the ambient temp. This includes water cooling setups, which use air to cool the water as it goes through the radiator. Those people who had 18ºC idle temps were sitting in a pretty cool room to get that.