One area that you will notice an improvement in is the fuel economy of your car. The best increase will be in vehicles with full performance exhausts, but even cat-back systems can help. The free-flowing exhaust allows the gases to flow out of the engine faster and easier. While the gains may not be earth-shattering, every little bit can help when you are talking about fuel economy.
While it may not be a benefit to everyone, the deeper tone and the more aggressive sound from the exhaust are often very appealing to the car owner. When you are selecting mufflers for the system, you can find highflow models with low tones that sound nice when the car is accelerating hard. Keep in mind that muffler shops will not build a custom system with mufflers that are not legal for the streets, so there may be some limitations in this area.
Custom coatings are often used to reduce heat under the car, protect the exhaust, and make the exhaust system look good. For show cars, stainless steel pipes are sometimes used, and the pipes are polished to a bright shine. These pipes are not typical on street cars, but they could be installed on one if you would like that.