But did you _actually_ try the alternatives?
I've been using mumble for more than a decade now, and I've pushing it into 3 working places as a substitute to skype, asterix and lync (the microsoft voip solution).
It's not really oriented for 1-1 conversation, which is odd to use at first, but once people start to use channels and the built-in chat you hardly go back.
The latency and voice quality is stunning, even across lousy links. There's simply no match. The built-in mic wizard is also well designed. If people follow the setup correctly, you can have hour-long conversations across the globe with 10 people and it's absolutely fantastic. I work in research, and I've aggressively pushed to do long phone conferences over mumble for exactly this reason.
With some colleagues we even setup some channels that broadcast with PTT, and it's genuinely fun if you're spread across offices. You can decide if you want to participate by joining or not the right hierarchy. This is something is often done for lan parties, but turns out you can find other uses too ;)
Like I said, the main shortcoming is that mumble was never designed for 1-1 conversations as you would expect in skype. We hack it around by creating temporary channels. I've always meant to improve the UI in this regard, but we found that the temporary channel approach has advantages as well [you can invite other people in a call - for one].
With outdated hardware we still manage to hold >100 of conversations over a single server, as basically it's only network bound.
You have a point that mumble does only audio, but fortunately this is what I care the most. We (unfortunately) still have to use other software for desktop sharing, but generally still use mumble for audio.