I now have just under 6000 miles on my '11 Mustang GT and it is the most solid, trouble-free car I have ever owned. Granted, it's still basically new, but there is not a single rattle.
My GT is every bit as solid feeling and quiet (if not even more so) than my 335i Coupe.
That said, I think every Mustang made before 2010 is garbage and cannot represent the massive number of improvements Ford has made to the car in the last year. It's not fair to compare a 2006 328i to a 2010 M3 just as it's not fair to compare a 2007 Mustang GT to an '11 Mustang GT. TOTALLY different car.