Speaking of Major 7th intervals:

Here's a great example of using them in a context that might sound pretty or ugly, depending...

In the song Retrovertigo by Mr. Bungle, the bridge is played on xylophone by parallel Major 7th intervals. It always sounded pretty to me, and I never gave it much thought. When I finally transcribed the parts, I was surprised at how awful that xylophone line sounds by itself, without the rest of the music. Those parallel major 7ths come off by themselves as clashing and wrong to me. Yet...

https://www.youtube.com/watch?v=icgFjJjkP8s

That whole song (and the band in general) are impressive examples of sneaking in very dissonant notes that might twist your ear. Notice there's also some augmented stuff happening in this "beautiful" song (depending on your tastes of course).

Context. :)

-Carl.