I've done ABX tests multiple times, and the vast majority of the time AAC at 200+kbps was indistinguishable from the source (CD WAV), and even 128kbps was fine for a lot of tracks. That said, one or two tracks tripped up the encoder and could reliably be distinguished even at 320kbps (IIRC 20 trials with 100% success rate). Not enough that I'd notice in casual listening, but the difference was audible.
You also need to use a decent encoder - I compressed a video using Handbrake with a 256kbps AAC audio track a few months ago (faac/Windows), and noticed immediately that the audio was bad (I initially assumed the source was bad, but the FLAC track in the source sounded fine). Replaced it with a 192kbps AAC track using Quicktime/OSX and the quality was significantly better.
Yes, ALL lossy codecs have some "killer samples" -- the source material always matters.
It's a sad fact that there are so many sub-par AAC encoders around. Best to use either Apples implementation, or either of those from Fraunhofer -- FhgAAC (free version distributed with Winamp), or the Open Source FDK-AAC that was developed for Android.
Yes, Opus is fantastic -- been following it's development since the start due to Telecoms background.
For those of us that care and have software that plays it, it's great but AAC is still the next best thing for compatibility on a wide range of devices (whilst still beating MP3 for sound quality and file size) which makes ad-hoc sharing with non-techies possible.
Hopefully that will continue to improve as it grows in popularity.
For sharing, it's better to use FLAC originals anyway. For playback - Opus is supported by Rockobx, which is useful on Sansa players if you need something very portable (that's what I use sometimes), and is supported on Android too. On desktop systems it's not an issue. Any decent cross platform player can play Opus.
You also need to use a decent encoder - I compressed a video using Handbrake with a 256kbps AAC audio track a few months ago (faac/Windows), and noticed immediately that the audio was bad (I initially assumed the source was bad, but the FLAC track in the source sounded fine). Replaced it with a 192kbps AAC track using Quicktime/OSX and the quality was significantly better.