I've converted my music library to opus and it's all offline, on my phone and PC. It's the only way to listen to some things that get caught in licensing hell, and that's only going to get worse over time.
It's an open source codec. I convert from FLAC (lossless) to Opus (lossy), I couldn't hear much if any difference between FLAC and 128kbits opus, your mileage may vary, but it saves me 10x space, very useful for a big library on an SD card compared to lossless.
Reasonable audiophiles usually agree that 320kbps mp3s are as good ar raw/wav/FLAC. They're maybe twice the size of 128 ones IIRC, who everyone notice the drop of quality in. 160 is okay IMO for walkman or car territory.
You cannot compare kbps numbers of files encoded with different codecs and make assumptions on quality.
Each codec is able to compress differently good, so a 64kbps in opus sounds much better than 64kbps in mp3, for instance. 128 kbps in opus for stereo is plenty, quasi-lossless, while on other formats it is pretty bad.