I see you are in the UK, so this may be irrelevant ... I have been using
Vonage Voice over IP (SIP) service for over 18 months. The Vonage service provides a regular US phone number (unlike Skype's SIP service) so incoming calls are no issue. They even migrate existing traditional Bell numbers so it is not necessary to change phone numbers in most cases. Broadband telephony is becoming far more common here. Since I started with Vonage our local broadband provider and AT&T have also started offering the service in my local area too. (AT&T apparently has decided rather than watch their business erode away to join the VOIP groundswell).
I find the Vonage service to be quite good. Of course, a solid broadband connection is important, but that has not been an issue in my office or while traveling using client's office broadband connections. Typically the Vonage service and quality can not be distinguished from traditional telephone lines. There is also an add on called SoftPhone (a Java app that runs on either PC or Mac) that works pretty good, although not as good as the traditional Vonage service via the Cisco box with a standard telephone (but that may be more of a comment on my PC's headset than SoftPhone.) I typically use it from my home/office in Ohio, although I have used SoftPhone from Sweden and France with perfect results on a good broadband connection. Some hotel broadband connections I have used were not so good, suffering dropout during peak usage times within the hotel system (while still working well in the middle of the night / off peak times) Being able to place and
receive US calls from any broadband connection anywhere in the world at no additional cost is a big $ savings. BTW I also use Skype for communication with some European customers, and while it is quite good, it does not work as well as Vonage. More than once I have terminated a Skype call to a client in Europe only to immediately reconnect via Vonage without the same quality problems. But that may be a comment on the other person's broadband service ;-)
My telephony costs have reduced by more than 90% vs the local Bell and traditional long distance / international calling fees I had previously paid 8-)