I'm personally very pleased with the MS exhaust.
I couldn't really justify spending $400+ on a full catback for maybe 3-5 HP gains, but also wanted something that gave the 5 a more aggressive tone, and looked much better.
The MS exhaust is significantly louder than stock (as its intended to be), especially when flooring it or opening up the throttle alot.
It is, however, a very nice, DEEP, throaty sound. It not "tinny" and rattley, or whiney like you hear ricers have, and is not obnoxiously loud either.
I got used to the volume in a day or two, and since I always have my radio or CD player on, with the volume turned to the "third" bar, you cant really even hear it. Oh, and looks really nice as well. Not huge like some catbacks, but not teensy like the stock either.
Easily worth the $150 I paid for it new.