We had freetalk for a year, no problems at all. It worked out the box. Probably about half a dozen calls were rubbish quality during that time. Then Dixons realised it was making no money (and probably didnt know how to start billing those on an annual contract) - so sold out to Vonage.
I got the vonage box, lost my phone number for a day while they transferred and then got a terrible line. At work I've tried sipgate. Not so bad call quality, but 1 in 3 dials you get a number unobtainable sound as the sipgate network struggles to get you connected.
So, back to copper wires. It's the future?