I am very impressed with the way my MBRP system sounds. On the highway it is only slightly louder than stock, but when you get on it, it will get loud. I notice a slight drone when I am towing at lower RPM's - 1500-1700. When I am at 65-75mph the noise/drone is not at all a problem. I can clearly hear the turbo over the exhaust under all conditions.
I had an Aeroturbine on my last truck, the 4040L, and that was only slightly quieter than a straight pipe. I had a 6" tip on this that just barely cleared the body. Honestly, I think if I had left the stock exhaust pipe alone and not trimmed it for the tip, there would have been less in-cab noise.
The best analogy I could give you is that people could hear my CTD through my phone when I was driving my 04. 5. They used to ask me if my window was down. Now, in my 06, they can't tell I'm driving. Some of that can be attributed to the sound reduction upgrades that were made to the 06's, but the exhaust is definitely quieter.