I have a new MAC 4-2 exhaust system, but I'm dealing with some carb issues that are stopping me from being able to ride, so I can't offer any first hand thoughts.
From what I've hear from other people, the MAC slip ons are cheap, but fairly quiet compared to stock. If you're not looking for any huge performance gains, MAC is usually affordable and what I hear people go with...which is why I picked mine up.
Sorry I can't offer any more helpful information.