VOIP Phone service

Anyone have any input on Voice over IP phone service? The price for this service seems great but I just really question how well it works. I am starting to think it might be ok though as AT&T is now offering it. The prices I have found Start at $19.95 a month for unlimited calling within the I have great cable service it is rated at 256 out and 3000 in. From a good site I can get 300-350k per sec downloads. What I would like to know is what impact net usage would have on the phone and vice versa. I can live with Net service being slowed to half or even 1-4 speed when my wife is on the phone. For The saving I would get 64 out and 750 in would be an adaptable net speed. What I would not be able to live with is my wife getting choppy, garbled up or low volume phone service. Basically what I am saying is that the phone service has to work as well as it does with standard phone service and moderately impaired internet is ok when the phone is in use. Anyone have any experience with VOIP phone service on cable service?

Joe