Well, it was fine on the whole, but I did notice something holding it back: you had to push the volume a good few notches higher just to get the same output level as the 4 or 4S. Doing this caused the iPhone 5's on-screen volume display to turn a stress-inducing red color, which is arguably not what you need when you're trying to chill out to some chill-out.
ENGADGET: Smartphones for audiophiles: is the iPhone 5 more musical than its rivals?