I am planning my next exhaust system and have always thought it would be kind of cool to have one of these. Unfortunately I don't know anyone who has one, so: How long do they last? How well they seal? Are there any performance losses with it closed vs. not having one at all? Do you use it very often or do you forget it is on there? Which one do you have?

For any of you that just want to see what they are like: