SHJ commented that they are quieter for those outside, but I must admit that I have not noticed this. LHR-bound planes pass over my place at about 4,000 ft and although I don't mind noise I keep a mental note of different plane's sound. Weather, temperature and flightpath/angle all cause considerable variation, but 787s (on average) seem to me to be very comparable with many other twins, apart from some older ones which can be surprisingly loud!
Without wishing to get into tedious and partisan A/B comparisons, I have to say that the 380 does seem to be (on average) noticeably quieter and "smoother" sounding than other 4 holers, which does impress me, given its chunky looks.
I appreciate that I only really hear approach sound, and hve not heard enough take-offs to make any valid comments.
I have read PPRuNers' comments about the smoothness of 787 flights.