I checked out Vonage's cheap-ass plan. $10 a month for 200 minutes, $16 and some change after all the taxes and fees get tacked on. They claim you can take it with you but I didn't look into how that would work.
I've been on the other end of a few Vonage calls or what I assumed was some kind of IP telephony. Very, very poor quality, it was constantly cutting in and out and very choppy. I chalked that up to the person on the other end of the phone eating up bandwidth doing something else at the time. My basic cable plan runs at 3Mbps and handles audio pretty well.