I use vox and I am totally happy with the product. Haven't had any problems with it, and I find the call charges to be far better than many others out there. International calling is dirt cheap as well. the only down side is that when I am offline, so are my phones, and for some odd reasons I can't manage to get a stable internet connection.
Not sure on bandwidth, relatively heavy users on our line so we haven't ever cared what portion of that is Vox.
I'm using Vox. Its only as good as VOIP over ADSL can be. Don't rely on the VOX by itself. I made the mistake by advertising only my VOX number and then there was an issue with my adsl line and no Vox calls came through. Also calls made to Vox numbers are more expensive as far as I know. Vox outgoing calls are cheaper and seems to be reliable enough most of the time.