It's just amazing.
You can throw anything at it. No matter the sample rate and the bitrate and the output has good quality without any of that narrowband, wideband, ultra-wideband speex nonsense.
All that is missing is an "hybrid" mode just like wavpack that produces combined output of roughly the same size as flac.
re: wavpack, from wikipedia:
> WavPack also incorporates a "hybrid" mode which still provides the features of lossless compression, but it creates two files: a relatively small, high-quality, lossy file (.wv) that can be used by itself; and a "correction" file (.wvc) that, when combined with the lossy file, provides full lossless restoration. This allows the use of lossy and lossless codecs together.
This hybrid encoding format lets you keep files for both purposes (lossless for audio production and archiving, pre-encoded to HQ lossy for audio consumption) in less space than would be required to keep e.g. an ALAC file + a cached lossy M4A encoding of it, the way iTunes does when you specify "optimize tracks' space-usage for portable devices."
Such a hybrid design is ideal for e.g. the Internet Archive, where they want to keep lossless archival representations of files for historians to study, and to create encodings from; but also want to serve usable representations of those artifacts to users who care more about transfer-speed and bandwidth cost than perfect fidelity.
Other attempts include Dolby TrueHD and DTS HD - but both of those ended up larger than just separate FLAC and Opus files.
I made some very crude tests a few minutes ago and got about 15% size increase in total size (opus + diff.flac vs flac --best)
However, as a result, the difference between an Opus decode and the original will only get you lossless reconstruction if you decode with the same build of Opus on the same machine. Also, since Opus uses IIR filters that ring to infinity, even on the same machine you only get bit-exact results if you decode each file from the beginning. It won't work if you seek.
Really, if you're concerned about the disk space required for separate FLAC and Opus encodes, delete the Opus version. FLAC decoding is extremely fast (5x faster than Wavpack), and Opus encoding is pretty efficient, too (on the same order as Wavpack decoding).
It's not published yet, though I might. It's pretty simple, just grabs a static JSON manifest (which is generated by the shell script I run when I add music to the collection, which adds replaygain tags and transcodes) which includes basic metadata and the ReplayGain tags so that I can set that manually in the browser. Uses the standard rule for continuous play and replaygain: play whatever was in the view when you played the first song then shuffle play, when the song you're playing is followed by the next song or follows the previous song on the same album, apply album gain, else apply track gain.
I have yet to add dynamic features like cross-player play count tracking, and server-side play counts for the webapp. I think these would be a lot more work than I put in.
My Github is https://github.com/xorgy if you want to follow/check in to see if I publish it.
I have about 600GiB FLAC music from bandcamp, random publishers, and CDs. I then have, separately, the same directory structure but encoded as 96k and 128k Opus. Still all fits just fine on a mere TiB disk. In a couple years the difference between 96k and 128 in terms of space won't matter and I'll drop 96.
And yeah, it's very nice to have the FLACs when I want to timewarp, transcribe, analyze, or remix. I usually do that at home though, so I can splurge on the bandwidth.
WRT the Internet Archive, Wavpack hybrid decoding is probably too slow and too rare to bother including in any major browser. Even if they did, if lossy listening is a priority, a dedicated lossy codec will save them money in short order on bandwidth. Indeed, this is exactly how IA does things, they even have wav in addition to flac and two or three lossy versions of (almost?) every recording.
Then, you could actually do mixing in apps like DJay straight on your iPad, on a whim, sometimes—without having planned for it in advance—and make it straight through to mastering and uploading the result... unless you filled up your whole iPad with random videos/apps/etc. in the meantime, in which case the lossless versions would be gone, you'd have to work with the lossy copies, and then, when you get home, restore the lossless copies (or transfer the project to your PC) to master the encoding.
So, not so much when you want the lossless version as canonical (most times when you have a lot of space), but rather when you want the lossy version to be canonical, and the lossless version to be there as a sort of "optional file to enable mastering using this track" file, that's sometimes available and sometimes not.