Last week I upgraded to iOS 8.4 and it was painless. It was pretty fast and I've done it on 5 iOS devices without a glitch (so far.) But, like I always say, unless you're willing to risk some problems, I'd recommend waiting a couple of weeks just to see if other people report problems you can't live with - I've read about people having GPS problems after the upgrade. I haven't noticed this, but I don't use GPS a lot.
There's an Apple iBook with lots more info on 8.4: https://itunes.apple.com/us/book/iphone-user-guide-for-ios-8.4/id917482340?mt=11
Here's a brief on Apple Music, included with iOS 8.4, highlighting the features: http://www.guidingtech.com/45230/apple-music-explained/