I like the sound out of my exhaust... before the baffle in my muffler collapsed and it started rattling like a banshee (being fixed this week, by the way!)
My exhaust is custom. The pipes are up .25" from stock all the way from the headers to the muffler. There are two Mustang 3.8 cats where the stock cats usually sit (KuoH got cats from a 5.0 as I recall). There's a flex pipe just behind the join between the pipes, roughly where the third cat usually sits.
This gives me good flow, and a pretty good low-end grunt. Although it might not be to everyone's tastes, it does lower and louden the exhaust note a little on the highway... and the flex pipe does cause a certain amount of rattling under acceleration that manifests as a "hiss". Reminds me a little of the sound of a turbocharger... but no... no boost
This was relatively cheap. My stock cats were toast, as were my O2 sensors. Well, the left and center cats were toast... the right one was OK. Anyway, the whole custom job cost less than the center cat alone from a stock system. About a month later, that same setup successfully passed Missouri emissions test with better results than I had gotten out of stock a year previously.
Shortly after that is when I started getting the rattling muffler... it's definitely coming from the muffler and I plan to replace the whole thing. Of course, I got the salvage muffler and then stored the car for 18 months... but c'est la vie.
As I said, my system is not for everyone. KuoH got a similar system, as did Halistan (does he ever log in???)... actually, Halistan's the guy who got it done first when his 93 started having problems with the exhaust. I stayed with him for a weekend and we got my car done before he moved to Texas
I like the sound it gives... the extra power is nice (even if it's psychosomatic!) and really on the highway the engine noise / exhaust noise is no worse than my Volvo at the same speed that runs another 600-800rpm higher than my SVX.