Until two days ago, I had heard of the term and knew it was some music format, or perhaps speech encoding, but I had no idea if it was a legacy 90s format or what.
Hence the PSA because I'm blown away by the quality difference (for speech at least, I haven't tried music but Wikipedia says that blind tests also showed better bitrate:quality ratios for music). I've always been using mp3 at ~160 for music and 64kbps for speech (or sometimes AMR in the past) but those days are over now. AAC already should have made me upgrade, but the support is less good (e.g. Android has some lag issues with certain AAC files). Opus works perfectly fine everywhere so far, does great compression on both speech and music, so now I have zero reason to ever go with anything other than MP3 anymore.