I am using Callwithus for about a year now, with 2 DID numbers - Canada and UK. My experience has been quite satisfactory: customer service through email was very responsive, pricing is one of the best around. Call quality to UK mobile sometimes was not to my satisfaction, so I switched to premium tariff and never looked back (7 - 8 cents a minute became 10-12; Skype charges over 22 cents the last looked.
On the down side, this is a very small provider with rock-bottom prices, so they are not equipped for hand holding. If you need a lot of support on very basic issues, look elsewhere.