My experience says that this is a huge benefit I had a Verizon "World Phone" for about 3 months (until I couldn't deal with it any longer) and I had crystal clear reception at the top of the Great Wall of China, 100km from the nearest town, but a 5min call without being dropped in the NYC metro area was NEVER possible - not even once

Completely agree with RobertS975 by the way - having to give people a different number to use when you're travelling is practically impossible. SkypeIn, with my US mobile number forwarded to it when travelling, has worked well in the past but then there's the issue of text messages. Overall there isn't a seamless solution today, regardless of which approach you take.