I have been using MagicJack for over 1.5 years now and love it as calling USA & Canada is cheaper than calling domestic numbers, voice is clear and never had any problem. Only downside is it needs your PC to be ON to make calls, no problems for me as it's in the office hooked to a machine that is always on for various uses.
Last year in summers I found TK6000 and bought it too, $99 and no more to pay ever, connected directly to my network so no need to keep computer ON. Now called netTALK DUO
but with yearly payments. Tech support is much more superior to MagicJack, I had some initial problem, and their tech responded immediately, used TeamViewer to connect to my PC and fixed some configuration in the router which was blocking TK6000. Using it at home as lot of relatives live in USA and it's easy for my parents to use, they just have to pickup the cordless and dial, as they always did on the regular home phone.
I wish I can get something similar for UK/Europe.