You can listen to various exhausts here.
I've always had stock exhausts on sportsbikes and never felt the need to change. But the previous owner of the DRZ400 I bought in the States had done lots of work on it including changing the carbs and setting up the jets for a complete Yoshimura system. Not sure I'd have spent that much for the performance increase it gave myself, but I must say sounds very nice. It's also rather loud and certainly attracts attention in traffic, which must be a good thing. Though coming home to a quiet neighbourhood late at night sometimes I wished it had a volume control! :-[
