I ran a Mumble server for a while. On the server side, it has excellent quality and lower latency than Teamspeak or Ventrilo, but can consume more bandwidth -- all depending on how you set it up, of course. I never admin a TS or Vent server so I can't really make a comparison between the server feature set, but I never found anything lacking in Mumble at least with a small group of friends using the voice chat.
The client seems to be harder for end users to set up and get working well -- whereas TS and Vent mostly work "out of the box", Mumble requires quite a bit more configuration.