The PSU is one of, if not the most important part of your PC. It feeds every other component stable power. Depending on its overall performance (which can be difficult to measure accurately, by the way), your PSU plays a big part in determining reliability, so you have to be careful when it comes to picking the right one for your machine.
Besides good performance, a PSU should also have the protection features able to save your components, including the power supply itself, should something go wrong. In areas with an unstable mains electricity network, you'd also be wise to invest in a solid UPS (Uninterruptible Power Supply), which will protect everything behind it from brown outs, sudden power losses, spikes coming from the network and other unwanted situations.
Source: Tom's Hardware