Britwife- my parents have Vonage. It works out at about .03 a minute to call the UK from the US. They seem to like it and call me ALL THE TIME! They have low bills every month too, I would recommend it. I haven't actually seen the setup they have- I think they have a special phone that they use to call me. Sometimes I can't hear them all that well on it, but after they dial they just switch phones and its fine. It could also just be my parents- they are kind of quirky. If you have any specific questions, I can ask them for you.
For those of you in the UK- I don't know if they still have these at the post office, but I got a phone card that has a christmasy picture on it. (they said they will sell them until they run out). It lasts for 90 days, its 3p a minute to call the US and you can buy them for £5 each. A major bonus of this card is that its FREE on Saturdays to US landlines and US mobiles!!! Free calls are from 12 am-12 pm UK time. I was skeptical at first, but when you dial it says 'YOU HAVE UNLIMITED TIME FOR THIS CALL!'