Most people who drive cars don't really quite understand the difference between high octane and lower octane gas at the pump.
It's a false misconception that higher octane fuel will automatically increase fuel mileage and performance. In reality, it often does neither. Your car does need a certain octane of fuel to burn properly through it's combustion cycle. If the octane is too low, and the compression of the engine too high, then the fuel will explode prematurely, resulting what is commonly known as engine knocking, or pinging. This can typically be heard at partial throttle when the car is under load, like when it's going up a hill.
Knocking and pinging are hard on an engine, and can cause premature engine wear.
Most of today's modern cars have high compression engines, which would make you think that they would need high octane fuel. In reality, it often depends upon a variety of factors. For example, modern cars can easily correct for low octane fuel by electronically advancing or retarding the ignition timing. The computer detects the car knocking and compensates, which may result in a very small reduction in power that often goes unnoticed to the driver. Older cars typically don't have knock sensors, and may ping/knock more often because their fuel systems can't adjust for the lower octane fuels.
The bottom line? You only need to run high octane gas in your car if it pings and detonates on the lower octane stuff. I recommend that people try out the lower octane fuel for a few tanks and keep a close ear for engine knocking. If all is fine, then you don't need to waste your hard-earned money on higher octane fuel.