What's it good for?
Everything. ;)
You could consider it the successor to Vorbis, but it also includes a separate algorithm for speech to provide a single codec suitable for all purposes.
Which isn't to say it will choose one algorithm to encode a music audio file differently from a spoken audio file - it can switch per frame for optimal results.
Tech specs from the project web site:
Bitrates from 6 kb/s to 510 kb/s
Sampling rates from 8 kHz (narrowband) to 48 kHz (fullband)
Frame sizes from 2.5 ms to 60 ms
Support for both constant bitrate (CBR) and variable bitrate (VBR)
Audio bandwidth from narrowband to fullband
Support for speech and music
Support for mono and stereo
Support for up to 255 channels (multistream frames)
Dynamically adjustable bitrate, audio bandwidth, and frame size
Good loss robustness and packet loss concealment (PLC)
Floating point and fixed-point implementation
https://opus-codec.org
And it's a royalty-free open format etc.
It's a good choice for people who compress their music when they sync it to a phone because you can get transparency with lower bitrates to free up storage space. It should play fine on any Android phone from the past few years. Not sure about iPhones because... Apple.