Opus has different modes (Celt mode, Silk mode and a Hybrid mode): The first one has the best performance.
Newer versions of Opus are always backward compatible, so there will be no codec problems for example in Mumble as with Celt, Speex, Opus.
See Codecs in Mumble
Mumble uses the Opus codec since version 1.2.4.
We made few tests in Mumble, few subjective facts:
Another advantage (found on freenode in #mumble):