Like I said, it will work for SOME popple. But there are drawbacks.
If you get a power outage, your land-based phone will, in likelihood, still work. Your IP phone won't, since it won't have power.
If you have a cable modem, your phone service is at the mercy of the cable TV provider. Cable TV, for instance, is not considered a "lifeline" provider in case of natural emergencies. They aren't required to restore it as fast as, say, power and regular phone lines.
If you use DirecWay (DirecTV high-spped Internet), you will have "satellite lag" in all your calls. I'm sure you know what satellite lag is.
DSL speeds vary, and might not always be fast enough.
A spine problemo in your area will knock out not only your surfing but your phone as well.
As soon as IP Telephony hits the big time, the hackers will have a heyday. You think you're getting spammed now?
Now, VoIP will work best when in a LAN intra-office environment, removing the requirement for separate cabling runs from the office's switch to each user telephone. Instead, the phones will be ON the LAN along with the user's PC. Avaya (used to be Lucent) will have a Small Office VoIP out this spring that does this. I know, because phone systems are my bidness.
Michael
Something makes me think you would like
Herbert Kornfeld in Accounts Receivable