I recently got the RB exhaust and it is almost an exact duplicate in dimensions of the stock except for the exhaust tip which is considerably larger in diameter (I have the Sport version). The thing is beautiful to look at given its stainless steel construction. The mating flange is exactly the same and match up with the stock pipe; a perfect fit. The exhaust comes with 2 bolts and a gasket, a 10-15 mins install. The deep low baritone sound growls when pushed to over 4k rpms, but when cruising at highway speeds (70-80) it just hums. Not distracting or annoying. With the music on (not too loud), windows closed, A/C on, you barely noticed it. With the stock tires, I hear the road more than the exhaust.
The other thing I noticed is that the shifting seems easier. By that I mean the transition between gears seems smoother, the clutch engages better. I am not sure if the smoother air flow out the exhaust has anything to do with that. However it was something I noticed right away. When idling, the exhaust purrs and the sound upticks a bit with each cycling of the A/C (the A/C raises the rpms a bit). To me, it was worth the expense. The car sound a lot more European.