Modern engines use a device called a knock sensor to detect the rattling and vibration within a cylinder that signals preignition. These sensors send a signal to the vehicle's Engine Control Unit (ECU), sometimes called an Engine Control Module (ECM), which then adjusts the engine's timing -- when the spark plugs fire -- to reduce or prevent the knock. Because these sensors are so effective, cars with knock sensors rarely experience engine knock, even when running on regular unleaded gasoline. However, because the timing is slightly adjusted when low-octane fuel is present, the car's performance and fuel efficiency is diminished slightly, too.
Is the loss in performance dramatic enough to notice? For most people, the answer is probably no. According to one estimate, a car running on regular unleaded instead of premium will take about a half a second longer to accelerate from zero to 60 mph (97 kilometers per hour). Unless you're drag racing, it's unlikely that this will matter. Still, it's conceivable that you might find yourself in a situation where that half second just might matter -- merging safely with highway traffic, for instance. So, it's really up to you to decide if this is important.
Some experts claim that the loss in fuel efficiency when using regular gas in a luxury or high-performance car will be so great that it will actually cancel out the savings you get from buying cheaper low-octane gas. This point is debatable, but you should keep it in mind.
Another reason that people use premium unleaded fuel is that they believe it will keep their car's engine cleaner. This is because some fuel companies advertise that they add special detergents to their higher grades of gas. However, U.S. Environmental Protection Agency regulations require that all grades of gas have detergent in them. Yes, the premium grades often have more detergent, but the lower octane fuels can keep your engine clean, too. If you have reason to believe that an unusual amount of buildup has collected in your engine, simply buy a detergent additive at your local auto store and add it to the tank yourself.
So, unless your owner's manual says that your luxury car "requires" premium gas, it isn't going to hurt your car if you don't use it. And regular gas isn't going to have a significant impact on your engine's performance, either. Basically, it's up to you to decide whether the small impact that it does have is worth the extra cost of premium fuel.