Well, that is an easy one. You will experience loss of power at high r's. You will also notice a lot of oil all over your car. It will idle poorly. I dont think that I need to go on. The best thing I can tell you is to replace it.
If it's a small dent, it probably won't cause any problem. If it is large enough to restrict exhaust flow, the engine will run hotter due to the restriction. You should replace it or try to bend it back if it's restricting.