Free performance, however it can come at the cost of extra power consumption and potentially instability. I think the added power consumption is worth it as long as you don't go overboard with overvolting. I've also never had problems with my CPU overclocks, but I keep them on the low side. If I'm not mistaken, increasing CPU clock speed increases power consumption linearly, whereas changing the voltage will have an exponential effect on power consumption. Meaning that if you stay near stock volts, power consumption won't change much.