We've recently switched from Qwest phone service to Mediacom VoIP.
My memories of VoIP calling are from a few years ago, when it seemed like you were calling someone half-way around the world. Long pauses, dropped parts of conversation and really, only way way communication..walkie-talkie style.
I'd have to say that I've been pleasantly surprised with the VoIP service Mediacom is offering. I can tell no difference between it and the standard copper line. The other bonus is that domestic long distance carries no additional charge. This is especially nice for me as my wife has family in other states that she likes to call 2-3 times a week.
Couple the VoIP phone service with Mediacom cable and HSI for 99 bucks and you really can't go wrong.
As an additional note I am somewhat disappointed with Mediacom's channel selection with their basic digital package as it doesn't have Noggin-obviously not a big deal unless you have young children. They were also out of DVRs when we switched to their cable service, nothing worse than having to watch commercials. :-(