Re: mp3, Opus is a pretty big improvement. In my experience I've found that Opus is pretty reliably transparent at 96kbit/s, whereas LAME (which really squeezes mp3 for all it's got) needs ~130kbit/s

I think encode costs are higher (although I'm struggling to find any actual data) but in my experience this is totally negligible in practice.

Of course, don't go converting all your mp3s to Opus, but if you're running a music streaming server or whatever, Opus is pretty sweet... and probably close to "as good as it gets" until/unless we figure out some crazy new tricks.