[gcp]

Skype used the SILK part of Opus to replace iSAC quite a while ago. (SILK is in fact their second iteration of their own codec to improve upon/replace it)

I couldn't find any direct comparison, it looks like the IETF standardization work didn't even consider it worthwhile comparing to iSAC as it was judged outdated and "being phased out". (For example, Google participated in the tests, but they only tested against iLBC, not iSAC)

So it's likely that Opus is way better. It can certainly scale to much higher quality than iSAC can just by the format alone.

Qualitywise, Opus outperforms the state-of-the-art high-latency codec at music encoding while being a low latency music & speech codec itself. The only case where another codec outperforms Opus anywhere seems to be AMR, when encoding speech at very low bitrates (<= 6-12kbps). But AMR also has higher delay.

You can read the summary here: http://tools.ietf.org/html/draft-ietf-codec-results-01

[ksec]

Opus, A lot better, it is a rather strange beast that it nearly manage to outperform in every category. With a bit more tuning work for a year or two it could properly be on the same level with AAC @ 256Kbps.

[gcp]

Do you have samples where AAC@256kbps outperforms Opus@256kbps? That'd be very interesting...