My folks had a Mustang in the '70s was these, and there's a downside to the design, aside from the sound.
It seems that moisture collects in the packing - more so than on a conventional muffler with metal baffling. If you're doing short trips (< 10 miles) there isn't enough heat produced to dry them out. My mother had a short commute of about five miles, so little rust holes kept showing up on the casing of the mufflers. While the exhaust sounded nice and deep when the glasspacks were new and solid, the note turned rather flatulent at idle once the holes appeared.
My father had them replaced once, but then the new ones started rusting through in about two years (this was in California, where hardly anything rusts and it's not unusual to see vehicles that are decades old with their factory mufflers still intact). Figuring it was a losing propositon for the way the car was used, he had the muffler shop put a stock system back on the car.
